Air Filter Replacement Interval Calculator
ANA›Life Services Authority›National Calculator Authority›Air Filter Replacement Interval 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 Replacement Interval Calculator
Estimate the recommended air filter replacement interval for your HVAC system based on filter MERV rating, household occupancy, pets, smokers, and local air quality.
Filter Type / MERV Rating
Fiberglass / MERV 1–4 (base interval: 30 days) Pleated / MERV 5–8 (base interval: 90 days) High-Efficiency Pleated / MERV 9–12 (base interval: 180 days) HEPA / MERV 13–16 (base interval: 360 days)
Higher MERV = finer filtration but may clog faster in dusty conditions.
Number of Occupants
More people → more skin cells, dust, and activity.
Number of Pets (dogs/cats)
Each pet adds significant dander and hair load.
Smokers in Household
Smoke particles clog filters rapidly.
Allergy / Asthma Sufferers
None Yes – recommend more frequent changes
Allergy sufferers benefit from cleaner filters.
Outdoor Air Quality
Good (rural / low pollution) Moderate (suburban) Poor (urban / high traffic / wildfire-prone)
Poor outdoor air increases indoor particulate load.
Average Daily HVAC Runtime (hours)
More runtime = more air cycled through the filter.
Calculate Replacement Interval
function airCalc() { // --- Read inputs --- var baseInterval = parseFloat(document.getElementById('air-filter-type').value); var occupants = parseFloat(document.getElementById('air-occupants').value); var pets = parseFloat(document.getElementById('air-pets').value); var smokers = parseFloat(document.getElementById('air-smokers').value); var allergyFactor = parseFloat(document.getElementById('air-allergy').value); var aqFactor = parseFloat(document.getElementById('air-aq').value); var runtime = parseFloat(document.getElementById('air-runtime').value);
var resultDiv = document.getElementById('air-result');
// --- Validation --- if (isNaN(occupants) || occupants 20) { resultDiv.style.display = 'block'; resultDiv.innerHTML = 'Please enter a valid number of occupants (0–20).'; return; } if (isNaN(pets) || pets 10) { resultDiv.style.display = 'block'; resultDiv.innerHTML = 'Please enter a valid number of pets (0–10).'; return; } if (isNaN(smokers) || smokers 10) { resultDiv.style.display = 'block'; resultDiv.innerHTML = 'Please enter a valid number of smokers (0–10).'; return; } if (isNaN(runtime) || runtime 24) { resultDiv.style.display = 'block'; resultDiv.innerHTML = 'Please enter a valid daily runtime between 1 and 24 hours.'; return; }
/ * ─── FORMULA ──────────────────────────────────────────────────────────────── * * Step 1 – Occupancy penalty factor * F_occ = 1 - (occupants - 1) × 0.04 (each extra person beyond 1 reduces interval 4%) * Clamped to [0.50, 1.00] * * Step 2 – Pet penalty factor * F_pet = 1 - pets × 0.15 (each pet reduces interval 15%) * Clamped to [0.25, 1.00] * * Step 3 – Smoker penalty factor * F_smoke = 1 - smokers × 0.25 (each smoker reduces interval 25%) * Clamped to [0.10, 1.00] * * Step 4 – Runtime normalisation factor * Standard reference runtime = 8 h/day * F_run = 8 / runtime (more runtime → shorter interval) * Clamped to [0.33, 2.00] * * Step 5 – Adjusted interval * Interval (days) = baseInterval * × F_occ * × F_pet * × F_smoke * × allergyFactor (0.75 if allergy sufferers present) * × aqFactor (0.65–1.00 based on outdoor AQ) * × F_run * * Step 6 – Round to nearest 5 days; enforce minimum of 15 days. * ──────────────────────────────────────────────────────────────────────────── /
var F_occ = Math.max(0.50, Math.min(1.00, 1 - (occupants - 1) * 0.04)); var F_pet = Math.max(0.25, Math.min(1.00, 1 - pets * 0.15)); var F_smoke = Math.max(0.10, Math.min(1.00, 1 - smokers * 0.25)); var F_run = Math.max(0.33, Math.min(2.00, 8 / runtime));
var rawDays = baseInterval * F_occ * F_pet * F_smoke * allergyFactor * aqFactor * F_run;
// Round to nearest 5, minimum 15 days var days = Math.max(15, Math.round(rawDays / 5) * 5);
// Convert to weeks / months for display var weeks = (days / 7).toFixed(1); var months = (days / 30.44).toFixed(1);
// Urgency colour var colour = days ' + r.label + '' + r.value + ''; }).join('');
resultDiv.style.display = 'block'; resultDiv.innerHTML = '### Recommended Replacement Interval ' + '' + days + ' days' + '
' + '≈ ' + weeks + ' weeks | ≈ ' + months + ' months
' + '' + urgency + '
' + '#### Factor Breakdown ' + '' + '' + 'Factor' + 'Value' + '' + '' + rows + '' + '' + '' + 'Tip: Check your filter visually every 30 days regardless of the calculated interval. ' + 'Replace immediately if it appears grey/clogged or if airflow noticeably decreases.' + '
'; }
#### Formula
Interval (days) = BaseInterval × Focc × Fpet × Fsmoke × Fallergy × FAQ × Frun
- BaseInterval – manufacturer-rated service life in days for the selected MERV tier (30 / 90 / 180 / 360 days).
- Focc = max(0.50, 1 − (occupants − 1) × 0.04) — each additional occupant beyond one reduces the interval by 4 %.
- Fpet = max(0.25, 1 − pets × 0.15) — each pet reduces the interval by 15 % (dander and hair).
- Fsmoke = max(0.10, 1 − smokers × 0.25) — each smoker reduces the interval by 25 % (fine particulates).
- Fallergy = 0.75 if allergy/asthma sufferers are present, else 1.0.
- FAQ = 1.00 (good) / 0.85 (moderate) / 0.65 (poor) outdoor air quality.
- Frun = clamp(8 / runtime, 0.33, 2.00) — normalises against an 8 h/day reference; longer daily runtime shortens the interval proportionally.
#### Assumptions & References
- This calculator provides an estimate. Always follow the filter manufacturer's maximum rated service life and inspect filters visually each month.
- References: ASHRAE Standard 52.2-2017; EPA Indoor Air Quality Guide; American Lung Association Health Effects of Indoor Air Pollution.
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