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AC capacity at outdoor temperature calculator
An air conditioner's rated capacity is measured at 95°F outdoor, with indoor air at 80°F dry bulb and 67°F wet bulb. On a 105°F afternoon it delivers less. A 3 ton unit rated at 36,000 BTU/h comes to about 34,040 BTU/h, 2.84 tons, a loss of 5.4 percent. At 115°F it is about 31,970 BTU/h, 11.2 percent down. This page scales the rating to the outdoor temperature and the indoor wet bulb you enter, with the NREL curve checked against a Goodman expanded performance table. It is an estimate for when the manufacturer's table is not at hand. To pick a unit against a load, use the Manual S calculator. For the design temperature at your ZIP code, use the HVAC design temperatures page.
- Take the total cooling capacity of the outdoor unit and coil pair at the 95°F rating point from the spec sheet or the AHRI certificate, in BTU/h. A 3 ton unit is about 36,000 BTU/h. The label tonnage is rounded, so use the printed capacity when you have it.
- Pick single-stage or two-stage, then enter the outdoor dry bulb, such as the cooling design temperature for the site, and the wet bulb of the air entering the indoor coil. The rating uses 67°F wet bulb. Indoor air at 75°F and 50 percent RH is nearer 62.6°F.
- The result shows the multiplier, the capacity in BTU/h and tons, and the change from the rating, with a small table at 85, 95, 105 and 115°F. Compare total capacity with the total load, then open the manufacturer's expanded performance table for the pair to confirm the figure and to read the sensible capacity.
Example result for the default inputs
Single-stage, rated 36,000 BTU/h, 105°F outdoor, 67°F entering wet bulb.
Total cooling capacity at 105°F outdoor
34,040 BTU/h
In tons
2.84 tons
Change from the rated capacity
-1,960 BTU/h (-5.4%)
| Rated capacity, 95°F outdoor, 80/67°F indoor | 36,000 BTU/h |
|---|---|
| Compressor | Single-stage |
| Outdoor dry bulb | 105°F |
| Indoor entering wet bulb | 67°F |
| Capacity multiplier | 0.946 |
| At 85°F outdoor, 67°F wet bulb | 37,840 BTU/h (multiplier 1.051) |
| At 95°F outdoor, 67°F wet bulb | 36,000 BTU/h (multiplier 1.000) |
| At 105°F outdoor, 67°F wet bulb | 34,040 BTU/h (multiplier 0.946) |
| At 115°F outdoor, 67°F wet bulb | 31,970 BTU/h (multiplier 0.888) |
- This is an estimate from a standard curve, for use when the manufacturer's expanded performance data are not at hand. ACCA Manual S selects equipment from those tables for the exact outdoor and indoor unit pair. Where you have them, read the capacity there and use this page only to check it.
- Total capacity only. Sensible capacity and the sensible heat ratio need the manufacturer's table, since they depend on the coil and the airflow. At a low sensible heat ratio the sensible capacity is much less than the total shown here.
- Assumes the rated airflow across the coil, 80°F indoor dry bulb and sea level, and no change from line set length, charge or a dirty coil.
For estimation purposes only. Consult a licensed engineer for final designs.
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How the capacity at an outdoor temperature is calculated
An air conditioner's rated capacity is measured at one condition: 95°F outside, and indoor air at 80°F dry bulb and 67°F wet bulb. On a hotter afternoon the condenser rejects heat at a higher temperature and the compressor delivers less, and a wetter or drier indoor return moves the capacity too. The NREL residential air conditioner model captures both with one curve in the outdoor dry bulb and the wet bulb of the air entering the indoor coil. The curve is divided by its own value at the rating point, which gives a multiplier. Rated capacity times the multiplier is the estimate.
- Total capacity curve
CAP-FT(EWB, ODB) = a + b EWB + c EWB^2 + d ODB + e ODB^2 + f EWB ODBTemperatures in °F, EWB the wet bulb of the air entering the indoor coil and ODB the outdoor dry bulb. NREL/TP-5500-56354 (Cutler et al. 2013) gives this biquadratic form in Equation 7 and the single-stage air conditioner coefficients in Table 7: a = 3.670270705, b = -0.098652414, c = 0.000955906, d = 0.006552414, e = -0.0000156, f = -0.000131877. The two-stage high stage has its own set in the same table.- Multiplier
multiplier = CAP-FT(EWB, ODB) / CAP-FT(67, 95)The report normalizes the curve to the rated conditions, 95°F outdoor and 67°F wet bulb at 80°F dry bulb. At the rating point the multiplier is exactly 1. At 105°F outdoor and 67°F wet bulb it is 0.946.- Capacity at the condition
capacity = rated capacity x multiplierFor 36,000 BTU/h rated and a multiplier of 0.946, the capacity at 105°F is 34,040 BTU/h. The result is shown to the nearest 10 BTU/h, since a fit of this kind is not good to the last BTU.- Change from the rating
change = capacity - rated capacity; percent = (multiplier - 1) x 100A loss of 1,960 BTU/h, 5.4 percent, for the example. Below 95°F outdoor the multiplier is above 1 and the page shows a gain: 37,840 BTU/h at 85°F, 5.1 percent up.- Tons
tons = BTU/h / 12,000One ton of cooling is 12,000 BTU/h. The 34,040 BTU/h above is 2.84 tons.- Wet bulb of the indoor air
EWB from indoor dry bulb and relative humidity (ASHRAE psychrometrics)The curve takes the wet bulb, not the dry bulb. Air at 75°F and 50 percent RH has a wet bulb of about 62.6°F. With that in place of 67°F the multiplier at 105°F outdoor is 0.895, which makes 32,230 BTU/h for the example.- Two-stage, high stage
same form, coefficients from the two-stage high column of Table 7At 67°F wet bulb and 115°F outdoor the single-stage multiplier is 0.888 and the two-stage high stage is 0.896. The low stage, variable speed and inverter units are not covered.
Worked examples against published data
The Goodman GSX14 sheet (SS-GSX14, 12/19) prints an expanded cooling table for each outdoor unit and coil pair. For the GSX140361 with a CA*F3642 coil, at 80°F indoor dry bulb and 1,200 CFM, the sheet gives 34.2 MBh at 95°F outdoor and 67°F wet bulb. The table below takes that figure as the rating, scales it with the curve, and sets the result beside the printed value. Each cell reads printed MBh, then the calculated MBh and the difference. Along 67°F wet bulb the curve is within 2.4 percent. Away from it the curve runs steeper in wet bulb than this unit, by up to 6.6 percent at 71°F wet bulb and 75°F outdoor.
| Outdoor, °F | 63°F wet bulb | 67°F wet bulb | 71°F wet bulb |
|---|---|---|---|
| 75 | 35.6 / 35.4 (-0.5%) | 36.7 / 37.6 (+2.4%) | 38.3 / 40.8 (+6.6%) |
| 85 | 34.7 / 34.0 (-2.1%) | 35.8 / 36.0 (+0.4%) | 37.4 / 39.0 (+4.2%) |
| 95 | 33.2 / 32.4 (-2.4%) | 34.2 / 34.2 (0.0%) | 35.8 / 37.1 (+3.5%) |
| 105 | 31.2 / 30.7 (-1.5%) | 32.3 / 32.3 (+0.1%) | 33.9 / 35.0 (+3.3%) |
| 115 | 29.5 / 28.9 (-1.9%) | 30.5 / 30.4 (-0.4%) | 32.1 / 32.9 (+2.4%) |
Eight cases below run through the same functions the form uses. The first is the rating point itself. Three use the Goodman pair and show the printed capacity beside the estimate. The 3 ton rows show what a hotter afternoon, a lower indoor wet bulb and a mild day do to the same unit.
| Case | Setup | Result |
|---|---|---|
| At the rating point, the capacity is the rating | Single-stage, rated 36,000 BTU/h, 95°F outdoor, 67°F wet bulb | 36,000 BTU/h (3.00 tons), 0.0 percent, multiplier 1.000 |
| Goodman GSX140361 pair, 105°F afternoon | Single-stage, rated 34,200 BTU/h, 105°F outdoor, 67°F wet bulb | 32,340 BTU/h (2.70 tons), -5.4 percent, multiplier 0.946, sheet prints 32,300 |
| Same pair at 115°F | Single-stage, rated 34,200 BTU/h, 115°F outdoor, 67°F wet bulb | 30,370 BTU/h (2.53 tons), -11.2 percent, multiplier 0.888, sheet prints 30,500 |
| Same pair at 105°F with a 63°F wet bulb | Single-stage, rated 34,200 BTU/h, 105°F outdoor, 63°F wet bulb | 30,720 BTU/h (2.56 tons), -10.2 percent, multiplier 0.898, sheet prints 31,200 |
| 3 ton unit at 105°F, indoor air at 75°F and 50 percent RH | Single-stage, rated 36,000 BTU/h, 105°F outdoor, 62.6°F wet bulb | 32,230 BTU/h (2.69 tons), -10.5 percent, multiplier 0.895 |
| 4 ton single-stage unit at 115°F | Single-stage, rated 48,000 BTU/h, 115°F outdoor, 67°F wet bulb | 42,630 BTU/h (3.55 tons), -11.2 percent, multiplier 0.888 |
| 5 ton two-stage unit at 110°F, high stage | Two-stage, high stage, rated 60,000 BTU/h, 110°F outdoor, 67°F wet bulb | 55,410 BTU/h (4.62 tons), -7.7 percent, multiplier 0.923 |
| 3 ton unit on a mild 85°F day | Single-stage, rated 36,000 BTU/h, 85°F outdoor, 67°F wet bulb | 37,840 BTU/h (3.15 tons), +5.1 percent, multiplier 1.051 |
The indoor wet bulb moves the answer. At 105°F outdoor the multiplier is 0.946 at 67°F wet bulb and 0.895 at 62.6°F, the wet bulb of 75°F air at 50percent RH. A drier return carries less heat into the coil, so the total capacity is lower. Sensible capacity needs the manufacturer's table.
Standards and data sources
- NREL/TP-5500-56354, Cutler, Winkler, Kruis, Christensen and Brandemuehl, Improved Modeling of Residential Air Conditioners and Heat Pumps for Energy Calculations (January 2013) - the capacity curve and its coefficients. Equation 7 is the biquadratic in the entering wet bulb and the outdoor dry bulb, Equation 13 scales the rated capacity by it, and Table 7 gives the air conditioner coefficients used here. The report collected manufacturer expanded performance tables for 260 air conditioners and 200 heat pumps from 5 manufacturers and took the standard curves, one set for single-stage units and one each for the low and high stage of two-stage units, from the manufacturer whose data were of the highest quality. NREL chose them so that they stand for the average performance of those air conditioners away from the rating point, judged by annual efficiency, not by capacity. The tables typically run from 75 to 115°F outdoor, which sets the range of this page. The Manual S selection tool in the Pro plan uses the heat pump cooling curve of the same report, and at 67°F wet bulb the two differ only in the third decimal.
- Goodman GSX14 split system air conditioner, Product Specifications SS-GSX14 (12/19) - an expanded performance table to check the curve against. For the GSX140361 with a CA*F3642 coil, at 80°F indoor dry bulb and 1,200 CFM, the sheet prints 34.2 MBh at 95°F outdoor and 67°F wet bulb, 32.3 MBh at 105°F and 30.5 MBh at 115°F. The curve is within 2.4 percent of the sheet along 67°F wet bulb from 75 to 115°F outdoor. The tests check 20 printed points.
- ACCA Manual S, Residential Equipment Selection - the rule this page does not replace. Equipment is selected from the manufacturer's expanded performance data at the design conditions, for the exact outdoor unit and indoor coil pair. The AHRI rating is a single test point, and this page estimates the move away from it when the table is not at hand.
- ASHRAE Handbook, Fundamentals, Chapter 1 (psychrometrics) - the wet bulb of indoor air from its dry bulb and relative humidity, through the same psychrometric engine as the psychrometric calculator. It gives about 62.6°F for 75°F and 50 percent RH.
Limitations of this calculator
- This is a standard curve from NREL, not your unit's table. Manufacturers publish expanded performance data for each outdoor unit and coil pair, and those numbers govern. ACCA Manual S selects equipment from them. Use this page when the table is not at hand, or to check a table you were given.
- The wet bulb dependence is steeper than the one Goodman table checked here. Along 67°F wet bulb the curve is within 2.4 percent of the sheet. Across all four wet bulbs from 85 to 115°F outdoor it is within 4.2 percent, and at 71°F wet bulb and 75°F outdoor it reads 6.6 percent high. Treat a result away from 67°F wet bulb as uncertain by more than the 2.4 percent along it: the worst point on that table is 6.6 percent off.
- Outdoor temperature runs from 75 to 125°F. The manufacturer tables behind the curves typically stop at 115°F, so from 115 to 125°F the result continues the curve and carries a warning. Below 75°F the page refuses.
- Entering wet bulb runs from 60 to 72°F. Below 60°F the single-stage curve turns upward as the wet bulb falls at high outdoor temperatures, which no coil does. Under 63°F the coil may run dry, and then total capacity equals sensible capacity. The curve is for a wet coil, so the page warns there.
- Total capacity only. Sensible capacity and the sensible heat ratio depend on the coil, the airflow and the unit, and need the manufacturer's table. When the load is mostly sensible, as it is in a dry climate, the sensible capacity is what has to cover it, and it is lower than the total shown here.
- The multiplier assumes the rated airflow, 80°F indoor dry bulb, sea level, the line set and charge the unit was rated with, and clean coils. Low airflow, a dirty coil, a long line set, an off charge and high altitude each cost capacity that this page does not subtract.
- Single-stage units, and the high stage of two-stage units. The low stage, variable speed and inverter units, and mini-split heat pumps, follow their own tables. A heat pump in cooling follows a curve of nearly the same shape, but its own table governs.
- Heat pump heating capacity is a different calculation, and is not covered here. The heat pump sizing calculator handles it.
- A capacity estimate is not a sizing result. The load that the unit has to meet also rises with the outdoor temperature. A licensed contractor, the manufacturer's data and the authority having jurisdiction decide the installation. The page makes no claim of code compliance.
Common mistakes with AC capacity at high temperature
- Using the rating point capacity on a hot afternoon. A 3 ton unit rated at 36,000 BTU/h delivers about 34,040 BTU/h at 105°F outdoor, 5.4 percent less, and about 31,970 BTU/h at 115°F, 11.2 percent less. The rating is a single test at 95°F, and a sizing check at a 105°F design day that still counts 36,000 BTU/h is counting capacity that is not there.
- Entering the indoor dry bulb as the wet bulb. The curve takes the wet bulb of the air entering the indoor coil. The rating point is 67°F wet bulb at 80°F dry bulb, and 80°F typed in the wet bulb field is refused. A house held at 75°F and 50 percent RH has a wet bulb of about 62.6°F, which at 105°F outdoor puts the multiplier at 0.895 and not 0.946.
- Counting total capacity against a sensible load. The number on this page is total capacity, sensible plus latent. Part of it goes to removing moisture. In a dry climate most of the load is sensible, so the unit can look big enough on total capacity and fall short on the sensible side. The sensible capacity needs the manufacturer's table.
- Taking the label tons as the rated capacity. A 3 ton label is 36,000 BTU/h on paper. The Goodman sheet prints 34,200 BTU/h at its rating point (95°F outdoor, 80°F dry bulb and 67°F wet bulb indoors) for the GSX140361 pair at 1,200 CFM, 5.0 percent under the label. Start from the printed capacity of the pair. Starting from the label carries that gap through every figure on the page.
- Treating the curve as the manufacturer's table. The curve is one standard set for every single-stage unit. Against one Goodman table it is within 2.4 percent along 67°F wet bulb, but it reads 6.6 percent high at 71°F wet bulb and 75°F outdoor. When a table exists for the pair, the table is the number to use, and ACCA Manual S says so.
- Using it for a variable speed unit. Inverter and variable speed units hold or boost capacity differently as the outdoor temperature rises, and many publish capacity at several compressor speeds. The single-stage and two-stage high curves here do not describe them. Use the manufacturer's expanded data for those.
Common questions about AC capacity at outdoor temperature
How much cooling capacity does an AC lose as the outdoor temperature rises?
About 5.4 percent at 105°F and 11.2 percent at 115°F, against the 95°F rating, for a single-stage unit on the standard curve at 67°F entering wet bulb. That is 0.54 to 0.56 percent of the rating for each degree above 95°F. A 3 ton unit rated at 36,000 BTU/h is about 34,040 BTU/h at 105°F and 31,970 BTU/h at 115°F. Your unit's table gives the exact figures.
What is a 3 ton AC's capacity at 110 degrees or 115 degrees?
For a 36,000 BTU/h single-stage unit at 67°F entering wet bulb, about 33,020 BTU/h at 110°F, a loss of 8.3 percent, and about 31,970 BTU/h at 115°F, a loss of 11.2 percent. At 125°F the multiplier falls to 0.827. The numbers come from a standard curve, so read the expanded performance table of your outdoor unit and coil for the exact capacity.
At what outdoor temperature is an air conditioner rated?
At 95°F outdoor dry bulb, with indoor air at 80°F dry bulb and 67°F wet bulb. The capacity on a spec sheet or AHRI certificate is the total capacity at that one condition. On a hotter day the unit delivers less, and on a cooler day it delivers more.
What is entering wet bulb and what value should I use?
It is the wet bulb temperature of the air that enters the indoor coil, which is the return air after it mixes with any outside air. The rating uses 67°F, with the dry bulb at 80°F. Air at 75°F and 50 percent RH has a wet bulb of about 62.6°F, and a humid house runs higher than 67°F. Use 67°F if you do not know, and the value for your design day if you do.
Does a two-stage air conditioner lose capacity the same way?
Nearly. In the NREL report a two-stage unit at the high stage has its own coefficients. At 67°F wet bulb and 115°F outdoor the single-stage multiplier is 0.888 and the two-stage high stage is 0.896. This page covers the high stage only, not the low stage, and not variable speed or inverter units.
How accurate is the estimate compared with a manufacturer table?
Along 67°F wet bulb the curve is within 2.4 percent of the Goodman GSX140361 table from 75 to 115°F outdoor. Across the four wet bulbs on that table it is within 4.2 percent from 85 to 115°F outdoor, and 6.6 percent high in the worst corner, 71°F wet bulb at 75°F outdoor. It is one standard curve for every single-stage unit, so a given unit can sit above or below it.
Does this give the sensible capacity?
No. It gives total capacity, sensible plus latent. The sensible capacity and sensible heat ratio depend on the coil and the airflow, and the manufacturer publishes them in the expanded performance table. If the load is mostly sensible, compare it with the sensible capacity from that table, which is lower than the total.
How is this different from the Manual S calculator?
The Manual S page explains how equipment is picked against a Manual J load and checked against the size limits, and the selector in the Pro plan runs that check for a specific AHRI-rated system. This page answers a narrower question: what a unit of a given rated capacity delivers at another outdoor temperature and wet bulb. The Manual S tool in the Pro plan applies the same NREL report, with the heat pump cooling curve at 67°F wet bulb, and the two differ only in the third decimal.
Can I use it for a heat pump in cooling mode?
As a rough check, yes. The NREL report gives a separate curve for heat pump cooling, and at 67°F wet bulb it differs from the air conditioner curve only in the third decimal. The manufacturer's table for the heat pump governs. Heating capacity is a different calculation and is not covered here.
Is there a limit on how many times I can run this?
No. This calculator runs in your browser with no daily limit and no signup.
Used on the same job
- BTU to tons converter Convert the capacity shown here between BTU/h, tons, kW and MBH.
- AC temperature split calculator Check the cooling a running unit gives from the return and supply temperatures.
- HVAC CFM calculator The airflow for the capacity at the design day, at the CFM per ton the coil is rated for.
Then confirm it against the manufacturer's table
The estimate tells you about how much capacity to expect on a hot day. Selecting the unit is another step, done from the expanded performance data for the exact outdoor unit and coil, where the sensible capacity is printed too. Our Manual S guide explains how that check works against the load, and the psychrometric calculator gives the indoor wet bulb. You can also ask the assistant to walk through the numbers with you. The unit on this page, 36,000 BTU/h, is an example, not a recommendation.
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