Air Filter MERV Rating Airflow Restriction Calculator

ANA›Life Services Authority›National Calculator Authority›Air Filter MERV Rating Airflow Restriction Calculator

.calc-container { max-width: 640px; margin: 2rem 0; padding: 1.5rem; background: #fff; border: 1px solid #ddd; border-radius: 8px; box-shadow: 0 1px 3px rgba(0,0,0,0.06); font-family: system-ui, -apple-system, sans-serif; } .calc-container h3 { font-family: Georgia, serif; font-size: 1.15rem; color: #1a1a1a; margin-bottom: 1rem; padding-bottom: 0.5rem; border-bottom: 2px solid var(--ac, #3d5a80); } .calc-row { display: flex; align-items: center; gap: 0.75rem; margin-bottom: 0.75rem; flex-wrap: wrap; } .calc-row label { min-width: 160px; font-size: 0.9rem; color: #333; font-weight: 500; } .calc-row input[type="number"], .calc-row select { flex: 1; min-width: 120px; max-width: 200px; padding: 0.5rem 0.6rem; border: 1px solid #ccc; border-radius: 4px; font-size: 0.9rem; font-family: system-ui, sans-serif; color: #1a1a1a; background: #fafaf8; } .calc-row input:focus, .calc-row select:focus { outline: none; border-color: var(--ac, #3d5a80); box-shadow: 0 0 0 2px rgba(26,74,138,0.12); } .calc-row .unit { font-size: 0.82rem; color: #888; min-width: 30px; } .calc-btn { display: inline-block; margin-top: 0.5rem; padding: 0.55rem 1.5rem; background: var(--ac, #3d5a80); color: #fff; border: none; border-radius: 4px; font-size: 0.9rem; font-weight: 600; cursor: pointer; font-family: system-ui, sans-serif; } .calc-btn:hover { opacity: 0.9; } .calc-result { margin-top: 1.25rem; padding: 1rem 1.25rem; background: #f0f6fc; border-left: 3px solid var(--ac, #3d5a80); border-radius: 0 6px 6px 0; display: none; } .calc-result.visible { display: block; } .calc-result-label { font-size: 0.78rem; text-transform: uppercase; letter-spacing: 0.06em; color: #666; margin-bottom: 0.25rem; } .calc-result-value { font-size: 1.6rem; font-weight: 700; color: var(--ac, #3d5a80); } .calc-result-detail { font-size: 0.85rem; color: #555; margin-top: 0.5rem; line-height: 1.5; } .calc-note { margin-top: 1rem; font-size: 0.8rem; color: #888; font-style: italic; } .calc-grid { display: grid; grid-template-columns: 1fr 1fr; gap: 0.75rem; margin-top: 0.75rem; } .calc-grid-item { padding: 0.6rem 0.8rem; background: #f8f9fa; border-radius: 4px; border: 1px solid #eee; } .calc-grid-item .label { font-size: 0.75rem; color: #888; text-transform: uppercase; letter-spacing: 0.04em; } .calc-grid-item .value { font-size: 1.1rem; font-weight: 600; color: #1a1a1a; } @media (max-width: 720px) { .calc-row { flex-direction: column; align-items: flex-start; gap: 0.3rem; } .calc-row label { min-width: auto; } .calc-row input[type="number"], .calc-row select { max-width: 100%; width: 100%; } .calc-grid { grid-template-columns: 1fr; } } .calc-chart { margin: 1rem 0; text-align: center; } .calc-chart svg { max-width: 100%; height: auto; } .calc-chart-legend { display: flex; flex-wrap: wrap; justify-content: center; gap: 0.6rem 1.2rem; margin-top: 0.6rem; font-size: 0.8rem; color: #555; } .calc-chart-legend span { display: inline-flex; align-items: center; gap: 0.3rem; } .calc-chart-legend i { display: inline-block; width: 10px; height: 10px; border-radius: 2px; font-style: normal; } .calc-related { max-width: 640px; margin: 2rem 0 1rem; padding: 1.25rem 1.5rem; background: #f8f9fa; border: 1px solid #e8e8e8; border-radius: 8px; } .calc-related h3 { font-family: Georgia, serif; font-size: 1rem; color: #1a1a1a; margin: 0 0 0.75rem; padding-bottom: 0.4rem; border-bottom: 2px solid var(--ac, #3d5a80); } .calc-related-list { list-style: none; padding: 0; margin: 0 0 0.75rem; display: grid; grid-template-columns: 1fr 1fr; gap: 0.4rem 1.5rem; } .calc-related-list li a { font-size: 0.88rem; color: var(--ac, #3d5a80); text-decoration: none; } .calc-related-list li a:hover { text-decoration: underline; } .calc-browse-all { margin: 0.5rem 0 0; font-size: 0.9rem; font-weight: 600; } .calc-browse-all a { color: var(--ac, #3d5a80); text-decoration: none; } .calc-browse-all a:hover { text-decoration: underline; } @media (max-width: 720px) { .calc-related-list { grid-template-columns: 1fr; } }

Air Filter MERV Rating Airflow Restriction Calculator

Estimate the pressure drop (inches of water column) across an HVAC air filter based on its MERV rating, face area, and airflow rate. Uses empirical pressure-drop coefficients derived from ASHRAE 52.2 filter performance data.

MERV Rating

MERV 1 MERV 2 MERV 3 MERV 4 MERV 5 MERV 6 MERV 7 MERV 8 MERV 9 MERV 10 MERV 11 MERV 12 MERV 13 MERV 14 MERV 15 MERV 16

Filter Width (inches)

Filter Height (inches)

Airflow Rate (CFM)

Filter Loading Condition

Clean (new filter) 25% Loaded 50% Loaded 75% Loaded (near end-of-life)

Calculate

function airCalc() { // --- Gather inputs --- var merv = parseInt(document.getElementById('air-merv').value); var width = parseFloat(document.getElementById('air-width').value); var height = parseFloat(document.getElementById('air-height').value); var cfm = parseFloat(document.getElementById('air-cfm').value); var loading = parseInt(document.getElementById('air-loading').value); var resultDiv = document.getElementById('air-result');

// --- Validation --- var errors = []; if (isNaN(width) || width 120) errors.push("Filter width seems unrealistically large (max 120 in)."); if (height > 120) errors.push("Filter height seems unrealistically large (max 120 in)."); if (cfm > 100000) errors.push("Airflow rate seems unrealistically large (max 100,000 CFM).");

if (errors.length > 0) { resultDiv.style.display = 'block'; resultDiv.innerHTML = 'Input Error:' + errors.join('') + ''; return; }

// --- Face Area --- // Convert inches to feet: area_ft2 = (W/12) * (H/12) var areaFt2 = (width / 12) * (height / 12);

// --- Face Velocity --- // V_face (fpm) = CFM / Area (ft²) var vFace = cfm / areaFt2;

// Warn if face velocity is outside typical HVAC range (200–750 fpm) var velocityWarning = ""; if (vFace 750) { velocityWarning = "⚠️ Face velocity exceeds recommended maximum of 750 fpm (" + vFace.toFixed(0) + " fpm). Increase filter size or reduce airflow."; }

// --- MERV Pressure-Drop Coefficients (Clean Filter) --- // Empirical initial resistance (inches w.c.) at 500 fpm face velocity, // derived from ASHRAE 52.2 / manufacturer data (typical mid-range values). // ΔP_clean = k_merv * (V_face / 500)^n // where n ≈ 1.7 for fibrous media (Darcy-Forchheimer approximation). // // k_merv values (in. w.c. at 500 fpm): var mervK = { 1: 0.03, 2: 0.04, 3: 0.05, 4: 0.06, 5: 0.07, 6: 0.09, 7: 0.11, 8: 0.14, 9: 0.17, 10: 0.20, 11: 0.24, 12: 0.28, 13: 0.33, 14: 0.40, 15: 0.50, 16: 0.65 };

// Final (loaded) resistance — typically 2× to 3× initial per ASHRAE/SMACNA. // The model treats end-of-life (100% loaded) as 2.5× clean for MERV 1-8, // and 2.0× for MERV 9-16 (denser media loads more uniformly). var mervFinalMult = (merv 0) { cfmActual = cfm * Math.sqrt(ratio); cfmReduction = cfm - cfmActual; } else { cfmActual = 0; cfmReduction = cfm; }

// --- MERV Description --- var mervDesc = { 1: "Pollen, dust mites, standing dust", 2: "Pollen, dust mites, standing dust", 3: "Pollen, dust mites, standing dust", 4: "Pollen, dust mites, standing dust", 5: "Mold spores, hair spray, fabric fibers", 6: "Mold spores, hair spray, fabric fibers", 7: "Mold spores, hair spray, fabric fibers", 8: "Mold spores, hair spray, fabric fibers", 9: "Legionella, humidifier dust, lead dust", 10: "Legionella, humidifier dust, lead dust", 11: "Auto emissions, welding fumes, coal dust", 12: "Auto emissions, welding fumes, coal dust", 13: "Bacteria, tobacco smoke, droplet nuclei", 14: "Bacteria, tobacco smoke, droplet nuclei", 15: "Bacteria, droplet nuclei (sneeze), all smoke", 16: "All combustion smoke, carbon dust, fine particles" };

var loadLabel = ["Clean (new)", "25% Loaded", "50% Loaded", "75% Loaded (near end-of-life)"][loading];

// --- Build Result HTML --- var html = '### Results '; if (velocityWarning) { html += '' + velocityWarning + '

'; }

html += ''; html += 'ParameterValue';

html += 'Filter Face Area' + areaFt2.toFixed(2) + ' ft²'; html += 'Face Velocity' + vFace.toFixed(1) + ' fpm'; html += 'MERV ' + merv + ' Captures' + mervDesc[merv] + ''; html += 'Clean Filter ΔP' + dpClean.toFixed(3) + ' in. w.c. (' + dpCleanPa.toFixed(1) + ' Pa)'; html += 'Current ΔP (' + loadLabel + ')' + dpCurrent.toFixed(3) + ' in. w.c. (' + dpCurrentPa.toFixed(1) + ' Pa)'; html += 'End-of-Life ΔP (fully loaded)' + dpFinal.toFixed(3) + ' in. w.c. (' + dpFinalPa.toFixed(1) + ' Pa)'; html += 'Airflow Restriction (vs. 0.5 in. w.c. system)' + restrictionPct.toFixed(1) + '%'; html += 'Estimated Actual Airflow' + cfmActual.toFixed(0) + ' CFM'; html += 'Estimated CFM Reduction' + cfmReduction.toFixed(0) + ' CFM (' + ((cfmReduction/cfm)*100).toFixed(1) + '%)'; html += '';

// Recommendation var rec = ""; if (dpCurrent >= dpFinal * 0.9) { rec = '🔴 Replace filter immediately. Pressure drop is at or near end-of-life threshold.

'; } else if (dpCurrent >= dpFinal * 0.6) { rec = '🟡 Plan filter replacement soon. Significant loading detected.

'; } else { rec = '🟢 Filter is within acceptable operating range.

'; } html += rec;

resultDiv.style.display = 'block'; resultDiv.innerHTML = html; }

#### Formulas Used

1. Filter Face Area: Aface = (W / 12) × (H / 12)   [ft²]

2. Face Velocity: Vface = CFM / Aface   [fpm]

3. Clean Filter Pressure Drop (Darcy-Forchheimer fibrous media model): ΔPclean = kMERV × (Vface / 500)1.7   [in. w.c.] where kMERV is the empirical resistance coefficient at 500 fpm (tabulated from ASHRAE 52.2 data), and the exponent 1.7 reflects the combined viscous and inertial flow regime through fibrous filter media.

4. Loaded Filter Pressure Drop: ΔPloaded = ΔPclean + fload × (ΔPfinal − ΔPclean) ΔPfinal = ΔPclean × Mfinal   (Mfinal = 2.5 for MERV 1–8; 2.0 for MERV 9–16)

5. Airflow Restriction & CFM Reduction (simplified fan-system curve): CFMactual = CFMrated × √(1 − ΔPfilter / Psystem) where Psystem = 0.5 in. w.c. (typical residential external static pressure baseline)

Unit Conversion: 1 in. w.c. = 249.089 Pa

#### Assumptions & References

More Calculators

Read Next

Study Time Planner Authority Network America › Life Services Authority › National Calculator Authority .calc-container { max-width: 640px;...

References

Related Authorities

Life Services Authority › Professional Services Authority › Trade Services Authority › United States Authority ›