Window leakage estimator form
Advanced physics-based estimator for airflow, ACH, thermal load, and cost.
Example data table
The table below shows one sample case to demonstrate how the estimator reports airflow, ACH, and thermal impact.
| Scenario | Windows | Size per Window | Gap Width | Pressure | ΔT | Airflow | Thermal Load | ACH |
|---|---|---|---|---|---|---|---|---|
| Reference room | 4 | 1.20 m × 1.50 m | 1.8 mm | 20 Pa | 14 °C | 345.06 m³/h | 1,619.95 W | 4.11 |
| Improved sealing | 4 | 1.20 m × 1.50 m | 0.8 mm | 20 Pa | 14 °C | 153.36 m³/h | 719.98 W | 1.83 |
Formula used
ELA = N × 2 × (W + H) × f × gWhere
N is window count, W and H are window dimensions, f is leakage ratio, and g is average gap width in meters.
v = √(2 × ΔP / ρ)Here
ΔP is pressure difference in pascals and ρ is air density.
Q = Cd × ELA × vCd is the discharge coefficient. The result is airflow in cubic meters per second.
P = ρ × Q × cp × |Tin - Tout|This converts infiltration airflow into heating loss or cooling gain magnitude in watts.
ACH = (Q × 3600) / VE = (P × t) / 1000V is room volume and t is operating time in hours.
How to use this calculator
- Enter the total number of similar windows you want to assess.
- Provide width and height for one window in meters.
- Enter the average gap width and choose a realistic leakage ratio between 0 and 1.
- Set the expected pressure difference, indoor and outdoor temperatures, and room dimensions.
- Enter daily operating hours, energy price, air density, and air specific heat values.
- Click Estimate Leakage to show the results above the form, including graph, ACH, thermal load, and costs.
- Use the CSV and PDF buttons to export the current calculation summary for reporting or comparison work.
Frequently asked questions
1) What does this calculator estimate?
It estimates effective leakage area, infiltration airflow, air changes per hour, and the resulting thermal load. It also converts that load into daily and annual energy use and cost.
2) Why does pressure difference matter so much?
Air leakage happens because pressure pushes air through openings. A larger pressure difference increases the leakage velocity, which raises airflow and usually increases heating loss or cooling gain.
3) What is the leakage ratio input?
Leakage ratio represents how much of the full window perimeter behaves like an active gap. It helps you avoid assuming every edge leaks equally along its full length.
4) Is this the same as a blower door test?
No. A blower door test measures actual building leakage under controlled conditions. This tool is an estimator that uses geometry, pressure, and temperature inputs to approximate performance.
5) Why are room dimensions included?
Room dimensions are used to calculate room volume. Volume lets the calculator express leakage as ACH, which is a useful way to compare ventilation intensity between spaces.
6) Can I enter negative outdoor temperatures?
Yes. The calculator accepts negative temperatures. It uses the temperature difference magnitude to estimate the leakage-driven thermal load and labels the result as heating loss or cooling gain.
7) Why is discharge coefficient needed?
Real leakage paths are not perfect openings. The discharge coefficient adjusts ideal flow behavior to better represent contraction, friction, and shape effects along actual cracks and gaps.
8) How can I reduce window leakage?
Improve weatherstripping, reseal trim joints, correct warped sashes, repair locking hardware, and reduce uncontrolled pressure differences. Smaller effective gaps lower airflow, ACH, thermal load, and yearly energy cost.