How to Calculate Air Leakage Rate

Air Leakage Rate Calculator (ACH50)

Enter the CFM (Cubic Feet per Minute) measured at 50 Pascals pressure.

The total volume of air inside the conditioned space of the building.

function calculateAirLeakage() { // Get inputs var cfmInput = document.getElementById('cfm50Input'); var volInput = document.getElementById('volumeInput'); var resultBox = document.getElementById('resultBox'); var achDisplay = document.getElementById('achResult'); var cfmDisplay = document.getElementById('cfmResult'); var ratingBadge = document.getElementById('ratingBadge'); var ratingDesc = document.getElementById('ratingDesc'); var cfm50 = parseFloat(cfmInput.value); var volume = parseFloat(volInput.value); // Validation if (isNaN(cfm50) || isNaN(volume) || volume <= 0 || cfm50 < 0) { alert("Please enter valid positive numbers for Fan Flow and Building Volume."); resultBox.style.display = "none"; return; } // Calculation Logic: ACH50 = (CFM50 * 60) / Volume var ach50 = (cfm50 * 60) / volume; // Round to 2 decimals var achFinal = Math.round(ach50 * 100) / 100; // Determine Rating var ratingText = ""; var ratingClass = ""; var descText = ""; if (achFinal <= 0.6) { ratingText = "Passive House Standard"; ratingClass = "rating-excellent"; descText = "Exceptional airtightness. Meets Passive House Institute standards. Requires mechanical ventilation (HRV/ERV)."; } else if (achFinal <= 3.0) { ratingText = "Energy Efficient"; ratingClass = "rating-good"; descText = "Good airtightness. Meets most modern energy codes (IECC 2015+ requires 3.0 or less in many zones)."; } else if (achFinal <= 5.0) { ratingText = "Average New Home"; ratingClass = "rating-average"; descText = "Standard construction. May have some drafts but meets older building codes."; } else { ratingText = "Leaky / Older Home"; ratingClass = "rating-poor"; descText = "Significant air leakage detected. High potential for energy loss and drafts. Air sealing recommended."; } // Update DOM achDisplay.innerHTML = achFinal + " ACH50"; cfmDisplay.innerHTML = cfm50; ratingBadge.innerHTML = ratingText; ratingBadge.className = "alr-rating " + ratingClass; ratingDesc.innerHTML = descText; resultBox.style.display = "block"; }

How to Calculate Air Leakage Rate (ACH50)

Understanding air leakage is critical for assessing a building's energy efficiency, durability, and indoor air quality. The standard metric used by energy auditors and HVAC professionals is ACH50, or Air Changes per Hour at 50 Pascals of pressure. This calculator helps you determine that rate based on Blower Door test results.

What is ACH50?

ACH50 measures how many times the entire volume of air inside a house is replaced by outside air in one hour, assuming the house is under a pressure difference of 50 Pascals (roughly equivalent to a 20 mph wind blowing against all sides of the house simultaneously). A lower number indicates a tighter, more energy-efficient home.

The Air Leakage Formula

To calculate the air leakage rate, you need two primary data points: the airflow measured by the Blower Door fan (CFM50) and the conditioned volume of the house.

ACH50 = (CFM50 × 60) / Building Volume

• CFM50: Cubic Feet per Minute of airflow required to maintain a 50 Pascal pressure difference. This is read directly from the Blower Door manometer.
• 60: Multiplier to convert "Minutes" (from CFM) into "Hours" (for ACH).
• Building Volume: The total cubic footage of the conditioned space (Length × Width × Height of all rooms).

Example Calculation

Let's assume you are auditing a standard 2,000 square foot home with 8-foot ceilings. You perform a Blower Door test and the manometer reads 1,800 CFM50.

1. Calculate Volume: 2,000 sq ft × 8 ft = 16,000 cubic feet.
2. Get CFM50: 1,800 CFM.
3. Apply Formula: (1,800 × 60) / 16,000
4. Math: 108,000 / 16,000 = 6.75 ACH50

In this example, the result is 6.75, which indicates a moderately leaky home typical of older construction.

Interpreting Your Results

Once you have your ACH50 number, you can compare it against building standards to determine the "tightness" of the building envelope.

ACH50 Score	Classification	Notes
≤ 0.6	Passive House	Extremely airtight. Mechanical ventilation is strictly required.
0.6 – 3.0	Energy Efficient	Meets IECC 2015/2018 codes for most climate zones. Very good performance.
3.0 – 5.0	Standard New Home	Acceptable for some codes, but leaves room for improvement in air sealing.
5.0 – 10.0	Average Existing Home	Typical of homes built before 2000. Likely has drafts and higher utility bills.
> 10.0	Very Leaky	Significant air infiltration. High priority for weatherization and air sealing.

Why Lower Isn't Always "Better" Without Ventilation

While a low ACH50 saves energy, a house that is too tight ("Build it tight, ventilate it right") must have a mechanical fresh air system, such as an ERV (Energy Recovery Ventilator) or HRV. If you seal an older home down below 3.0 ACH50 without adding ventilation, you risk indoor air quality issues and moisture buildup.