Mold Growth Risk Calculator
Assess the mold risk in any room based on indoor humidity and temperature.
Know when conditions are dangerous and how to prevent mold growth.
Mold is a fungus that grows on organic surfaces when the combination of moisture, warmth, and organic material is present. It can cause serious health problems — especially for people with allergies, asthma, or compromised immune systems.
The key factor is relative humidity:
Mold begins to grow when relative humidity stays above 60–70% for an extended period (24–48 hours or more). At above 80%, mold grows rapidly on most building materials within days.
Mold risk by humidity and temperature:
| Relative Humidity | Temperature | Risk Level | Timeline |
|---|---|---|---|
| Under 50% | Any | Very Low | Mold unlikely |
| 50–60% | Under 68°F (20°C) | Low | Unlikely, minimal risk |
| 50–60% | 68–77°F (20–25°C) | Moderate | Some risk over weeks |
| 60–70% | Any warm | High | Mold possible within days |
| 70–80% | Any | Very High | Mold likely within 24–48 hours |
| Above 80% | Any | Extreme | Mold grows rapidly |
Ideal indoor humidity range:
The EPA and ASHRAE recommend indoor humidity of 30–50% for health and comfort. This range is too dry for most mold species and still comfortable for human habitation.
Common mold-prone areas:
- Bathroom without exhaust fan
- Basement without dehumidifier
- Kitchen near sink or dishwasher
- Attic with inadequate ventilation
- Window sills with condensation
- Under carpets on concrete floors
- Behind refrigerators and washing machines
Health effects of mold exposure:
- Allergic reactions (sneezing, runny nose, red eyes)
- Asthma attacks and respiratory issues
- Skin irritation
- In high exposure: more serious respiratory conditions
- Black mold (Stachybotrys chartarum): rare but associated with severe symptoms
How to reduce humidity:
- Run exhaust fans in bathrooms and kitchens during and 20 min after use
- Use a dehumidifier in basement or humid rooms
- Fix leaks immediately — a dripping pipe creates ideal mold conditions
- Ensure clothes dryer exhausts outside, not into the attic
- Open windows when outdoor humidity is lower than indoor humidity
- Use air conditioning — it dehumidifies as it cools
When you find mold:
- Less than 10 sq ft: clean yourself with bleach solution (1 cup bleach per gallon water)
- More than 10 sq ft: hire a professional mold remediation company
- Always wear N95 mask, gloves, and eye protection when cleaning mold
- Fix the moisture source first — cleaning without fixing the source is temporary