Mold risk calculator
Mold needs moisture. Enter your indoor temperature andrelative humidity for a plain-English risk reading and a rough sense of how quickly mold could take hold.
Evidence-checked. Facts and figures on this page are verified against published standards and government sources (US EPA mold & moisture guidance (keep RH 30–50%, below 60%)). Last verified July 2026 — our methodology.
Mold-growth risk
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- Your reading
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- Time to growth
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- Temperature
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The EPA advises keeping indoor relative humidity below 60% — ideally 30–50% — because mold germinates on ordinary surfaces once the moisture available at the surface stays high for long enough. Warmth (roughly 77–86°F) speeds growth; cold slows but does not stop it. Remember that a cold wall, window, or corner is colder than the room, so its local humidity is higher than this whole-room reading — those spots grow mold first.
How humidity drives mold
Mold spores are always in the air; they only bloom when they find moisture. Sustained relative humidity above 60% — especially on cool surfaces where the local humidity is higher still — is the trigger, so the fix is almost always moisture control, not bleach. Bring the room back under 60% with ventilation and aright-sized dehumidifier, and dry any wet materials within 24–48 hours. To confirm your number is in the healthy zone, use thehumidity comfort checker, and learn the difference between surface fuzz and staining inmildew vs mold.