For estimation and sales-phase use. Use it to check a system on a service call or to set a duct design budget before a quote. Not a substitute for the manufacturer's data or a licensed engineer's review.

Maintained by the BuildSolver engineering team · Updated September 21, 2026

Calculator · Static pressure

Static pressure calculator

Check a running system against its rated maximum external static pressure, or work out how much static pressure the ducts get after the filter, coil, registers and grilles take their drops. Budget mode also returns the friction rate to carry into duct sizing.

Choose measured TESP against the rating, a static pressure budget for duct design, or pressure at a different airflow.
In measure mode, enter the supply and return readings, the rated maximum and the drop of any coil or filter the rating leaves out that sits between the cabinet and a probe. In budget mode, enter blower ESP at design airflow, total effective length, design airflow and each component drop.
The result shows total external static pressure as a share of the rating, or the available static pressure and friction rate to carry into duct sizing.

One system at a time. The mode sets what the three main fields mean. Each component drop field loads with an example value from BuildSolver's engine. Replace it with the manufacturer's drop at your airflow. If you clear a field, it stays blank.

What to calculate

The three number fields below switch their meaning with this choice.

Supply static pressure, in. w.c.

Measure mode takes the supply reading. Budget mode takes the external static pressure the blower table lists at your design airflow and speed. Fan law mode takes a pressure you measured at a known airflow.

Return static pressure, in. w.c.

In measure mode, enter the return reading. A manometer usually shows it as a negative number, and the calculator uses its absolute value either way. In budget mode, enter the longest supply run plus the longest return run, with fittings counted as equivalent length. In fan law mode, enter the airflow when you took the reading.

Rated maximum ESP, in. w.c.

Measure mode wants the maximum external static pressure from the rating plate or the manufacturer's blower table. Budget mode wants the airflow you read the blower ESP at. Fan law mode wants the airflow you want the pressure for.

Outside the rating, between the cabinet and a probe

The rated maximum applies at the equipment's own inlet and outlet, and the blower table notes say whether the rating includes the coil or filter. Pick a component here only when the rating leaves it out and your probe sits on its far side. A supply probe above an add-on cased coil on a gas furnace is one example. A return probe upstream of a filter rack outside the cabinet is another. A probe beyond the component reads lower than the cabinet by that component's drop, so the calculator adds the drop you enter below. This form opens on the worked example, which has a cased coil picked. Set it to nothing for probes at the equipment's own inlet and outlet, or the example's coil drop lands on your reading.

Filter pressure drop, in. w.c. (optional)

Required in budget mode. In measure mode it counts only when the filter is picked above as outside the rating, and is required then. Take the drop at your airflow from the filter data. BuildSolver's engine uses 0.10 as a typical value. Enter 0 when the filter lives in a return grille.

Coil pressure drop, in. w.c. (optional)

Required in budget mode. In measure mode it counts only when the coil is picked above as outside the rating, and is required then. Take the drop at your airflow from the coil data. BuildSolver's engine uses 0.15 as a typical value for a coil outside the blower cabinet. Enter 0 in budget mode when the blower rating already includes the coil.

Example result for the default inputs

Supply 0.30 in. w.c. plus return 0.20 in. w.c., plus the coil drop outside the rating, compared with a 0.80 in. w.c. rated maximum.

External static pressure at the rating points

0.65 in. w.c.

Share of rated maximum

81 %

Calculation details
Supply static pressure	0.30 in. w.c.
Return static pressure, absolute value	0.20 in. w.c.
Measured total, supply plus return	0.50 in. w.c.
Coil drop outside the rating, added	0.15 in. w.c.
External static pressure at the rating points	0.65 in. w.c.
Rated maximum external static pressure	0.80 in. w.c.
Margin under the rated maximum	0.15 in. w.c.

For estimation purposes only. Consult a licensed engineer for final designs.

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How static pressure is calculated

Measure mode checks a running system against its rating, at the points the rating covers. Budget mode works the other way, starting from the blower table and handing a friction rate to duct design. Fan law mode moves a known reading to another airflow in the same duct system.

Total external static pressure: TESP = supply reading + |return reading|Both readings count as positive pressure against the blower. The return side reads negative on a manometer referenced to the room, so the calculator takes its absolute value.
Pressure at the rating points: ESP at rating = TESP + drops of components the rating leaves out that sit between the cabinet and a probeThe rated maximum applies at the equipment's inlet and outlet. A probe on the far side of a coil or filter the rating leaves out reads lower than the cabinet by that component's drop, so the drop you enter is added before the comparison. Pick nothing when your probes already sit at the rating points, since this form opens on the worked example with a cased coil picked.
Share of the rating: share = ESP at rating / rated maximum ESP x 100The rated maximum comes from the rating plate or the blower table for that model. The over-limit flag uses unrounded values. When a reading tops the rating by less than the rounding, the result shows extra decimals so you can see the overage.
Component total: filter + coil + supply registers + return grilles + balancing dampersEvery drop is your input. Use 0 for a component the system doesn't have or the blower rating already includes, and leave balancing dampers blank when there are none. A cleared filter, coil, register or grille field stops the calculation until you type a number.
Available static pressure: ASP = blower ESP at design airflow - component totalIf the component drops add up to the whole ESP or more, the calculator stops with an error. With no pressure left for the ducts, there is no friction rate to size to.
Friction rate: FR = ASP x 100 / total effective length (in. w.c. per 100 ft)The calculator checks the result against 0.05 to 0.15, the range BuildSolver's duct sizing calculator works inside. Outside it, duct sizing uses the nearest end of the range.
Pressure at another airflow: SP2 = SP1 x (CFM2 / CFM1)^2This is the system curve of a fixed duct system. Resistance rises with the square of airflow, and the relationship holds only while the ducts, dampers, filter loading and coil stay the same.

Worked examples for a service reading and a duct budget

The default reading is 0.30 in. w.c. on the supply and -0.20 in. w.c. on the return. Taken as magnitudes, that adds up to 0.50 in. w.c. The supply probe sits above an add-on cased coil the furnace rating doesn't include, and the furnace outlet below that coil sees more pressure than the probe by the coil's drop. Adding the 0.15 in. w.c. coil drop loaded as an example value gives 0.65 in. w.c. at the rating points, or 81% of a 0.80 in. w.c. rated maximum. Compared as read, the 0.50 in. w.c. total would show a 0.30 in. w.c. margin on paper. At the rating points the real margin is 0.15 in. w.c.

The table runs budget mode on four systems. Its component drops are the example values from BuildSolver's engine, with no product data behind them. Every row stays inside the 0.05 to 0.15 friction rate range, so none of them triggers a warning.

Available static pressure and friction rate for four example systems
System	Component total	Available static pressure	Friction rate per 100 ft
Furnace with cased coil, 0.80 ESP, 350 ft TEL	0.34 in. w.c.	0.46 in. w.c.	0.131 in. w.c.
Same furnace, filter drop entered at 0.20	0.44 in. w.c.	0.36 in. w.c.	0.103 in. w.c.
Air handler rated with its coil, 0.50 ESP, 250 ft TEL	0.19 in. w.c.	0.31 in. w.c.	0.124 in. w.c.
Furnace with cased coil, filter grille return, 0.70 ESP, 300 ft TEL	0.29 in. w.c.	0.41 in. w.c.	0.137 in. w.c.

The first two rows differ only in the filter. Raising the entered filter drop from 0.10 to 0.20 in. w.c. cuts the pressure left for the ducts from 0.46 to 0.36 in. w.c., and the friction rate falls from 0.131 to 0.103. Lower friction means larger ducts for the same airflow.

Fan law mode on a system reading 0.50 in. w.c. at 1,000 CFM puts the same ducts at 0.72 in. w.c. for 1,200 CFM.

Standards and data sources

Equipment rating plate and manufacturer blower table - the maximum external static pressure for measure mode, the ESP at your design airflow and speed for budget mode, and the table notes that say whether the rating includes the coil or filter.
Filter, coil, register and grille manufacturer data - the pressure drop of each component at your airflow, the inputs for budget mode and for any component outside the rating in measure mode.
BuildSolver Manual D engine - the example drops loaded in the form and used in the table (filter 0.10, coil outside the blower cabinet 0.15, supply registers 0.03, return grilles 0.03, filter grille 0.08, balancing dampers 0.03 in. w.c.), and the 0.05 to 0.15 friction rate range. No ACCA or manufacturer table backs these drops, so a field you clear stays blank and the calculator never swaps in a typical value.
ACCA Manual D, residential duct design - the duct design procedure the available static pressure and friction rate feed into. The formulas on this page are written the way the calculator applies them. The manual's own wording may differ.
CaptiveAire, Designing Air Flow Systems - the system constant K = S.P. / CFM squared and S.P. = CFM squared x K, the basis for fan law mode.

Limitations of this calculator

Readings are used as entered. The calculator does not correct for instrument error, altitude or air temperature.
Measure mode compares with the rating only after you tell it what sits between the cabinet and your probes. It can't see the equipment, so a wrong choice there, or a component drop that doesn't match the airflow, gives a wrong comparison.
Measure mode corrects for one case only, a component the rating leaves out with your probe beyond it. Some blower tables are rated without a filter or coil that sits inside the cabinet, between the probe points. The manufacturer's notes cover that case, and this calculator does not adjust for it.
The example drops in the form and the table are values BuildSolver's own engine uses as typical, with no ACCA or manufacturer table behind them. Replace them with the drops for your filter, coil, registers and grilles at your airflow.
Total effective length is entered directly. The duct sizing calculator builds its own estimate from the longest supply run times a fittings factor, so its friction rate can differ from this one. Carry the available static pressure across and let that page work out its own friction rate.
Fan law mode describes only the duct system. It tells you the pressure the ducts will need at a new airflow. Whether the blower can deliver that airflow at that pressure is in the blower table.
This page does not size ducts or check velocity. Room-by-room Manual D is part of the Pro and Team plans.

Common static pressure mistakes

Adding the return reading with its minus sign. Supply 0.50 plus return -0.25 is not 0.25. Both sides resist the blower, so the total is 0.75. Add the magnitudes.
Comparing readings with a rating taken at different points. A furnace rating stops at the furnace outlet. Read supply pressure above an add-on cased coil and the probe sees less than the furnace outlet does, short by the coil's drop. Measure between the furnace and the coil, or add the coil drop from the coil data before you compare.
Handing the blower's full ESP to duct design. The filter, coil, registers and grilles take their share first. Sizing ducts to the full blower number overstates the friction rate, and the ducts come out too small for the pressure that is really left.
Leaving a coil out of the budget. When the blower table was rated without the coil, the coil's drop comes out of the budget. Enter 0 only when the table notes say the coil was in place during the rating.
Using the fan law to promise more airflow. The square law tells you how much pressure the same ducts need at more airflow. It says nothing about whether the blower has that pressure to give. Check the blower table at the new speed before you quote it.

Common questions about static pressure

How do I measure total external static pressure?

With the blower running at the speed you are checking, read static pressure on the supply side and on the return side, then add the two readings as positive numbers. Pick your test points to match the rating. If the installation instructions list test points, use those.

Why does my return reading show a negative number?

Return reads negative against the room. The calculator uses the absolute value, so -0.25 and 0.25 give the same total.

What if a coil or filter the rating leaves out sits between the cabinet and my probe?

Pick that component and enter its drop at your airflow from the manufacturer's data. A probe beyond the component reads lower than the cabinet by that drop, so the calculator adds it to your reading and compares the sum with the rated maximum. Nothing is added unless a component is picked, and the form opens with the example's cased coil picked, so switch it to nothing for probes at the rating points.

What static pressure is too high?

The equipment sets that limit. Compare the pressure at the rating points with the maximum external static pressure on the rating plate or in the blower table for that model. The calculator shows it as a share of that number and flags anything above it.

What is available static pressure?

It is the pressure left for the duct runs after the filter, coil, registers, grilles and dampers take their share of what the blower delivers at design airflow. Divide it by total effective length and multiply by 100 to get the friction rate the ducts are sized to.

Where do the example component drops come from?

They are values BuildSolver's Manual D engine uses as typical, 0.10 for a filter, 0.15 for a coil outside the blower cabinet, 0.03 for supply registers, 0.03 for return grilles and 0.03 for balancing dampers, in in. w.c. When no coil drop is given, the engine itself uses 0.00, and it only adds dampers on systems with five or more rooms. No standard or manufacturer publishes these numbers. Replace them with the drop from the filter, coil, register and grille data.

What if the friction rate lands outside the range?

The calculator still shows it and explains what happens next. BuildSolver's duct sizing calculator works between 0.05 and 0.15 in. w.c. per 100 ft. Below that it sizes at 0.05, which needs more pressure than the blower has left. Above it, it sizes at 0.15 and part of the pressure goes unused.

Can I predict static pressure at a higher blower speed?

For the same ducts, yes. Pressure rises with the square of airflow, so 20 percent more airflow needs 1.44 times the pressure. The blower table tells you whether the blower can actually reach that point.

Does the BuildSolver chat measure static pressure?

No. The chat has no static pressure tool, so the measurement and the budget come from this page. The chat does run the free simplified duct sizing, which takes the available static pressure from this page as one of its inputs.

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

Take the available static pressure into duct sizing

Give the assistant the available static pressure from budget mode, the design CFM, the longest supply run and the duct material, and it runs the free simplified duct sizing. Or enter the same inputs on the duct sizing calculator page.

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