How does the bathroom humidity calculator work?

It estimates how much water vapour a hot shower adds to your bathroom air and whether ventilation clears it before it condenses. From your room volume, climate humidity, shower frequency and current ventilation it models the peak post-shower humidity and dew point, then flags whether cooler surfaces like mirrors and tiles will sweat and seed mould. It returns a risk score out of 100.

What inputs do I need to use it?

Enter the bathroom length, width and height in feet, pick a climate band that matches your city (coastal or humid, moderate inland, or dry), the number of showers per day, and your current ventilation, whether none, an openable window or an exhaust fan. If you have a fan, add its CMH rating. Tick the windowless option if the bathroom has no daylight or openable window.

How much exhaust ventilation does a bathroom need?

A common planning target is roughly eight to fifteen air changes per hour, so a small bathroom often needs an exhaust fan of about 80 to 120 CMH, more in humid coastal homes or windowless rooms. Run the fan through the shower and for the run-on minutes the tool suggests. These are indicative figures, so confirm fan sizing, ducting and layout on site and per NBC 2016 Part 8 ventilation guidance.

Humidity & Mould

Bathroom Humidity Calculator

See how much steam your showers add and whether ventilation clears it before it condenses into mould. Get a risk score, the exhaust CMH you need and how long to run the fan. Indicative India 2026 — confirm on site.

Your bathroom

Length (ft)

Width (ft)

Height (ft)

Climate / ambient humidity

Showers per day

Current ventilation

Windowless bathroom (no daylight drying)

Model assumes ~28°C post-shower air and ~600 g of vapour per hot shower. Condensation forms when surfaces drop below the dew point (28°C at your peak). Fan CMH is cubic metres of air moved per hour — bigger rooms and humid climates need more.

Condensation & mould risk

0/100

High risk · main driver: ventilation gap · showers currently fog the room

Recommended exhaust

82 CMH

8 air changes/hr

Run fan after shower

23 min

then it can switch off

Peak humidity

100%

dew point 28°C

Surface condensation

Heavy / fogging

mirror & tiles sweat

What is driving your 78/100 score. Indicative — confirm ventilation and finishes on site.

Mould-prevention tips

Fit or upsize an exhaust fan to about 82 CMH and run it during every shower plus ~23 min after.
Wipe the mirror, tiles and cistern after showering — surfaces stay below dew point and sweat, seeding black mould.
Seal grout lines, fix dripping taps and inspect ceiling corners and silicone joints monthly for early black spotting.

Plan a dry, mould-free bathroom

Get a ventilation and finishes strategy for this room from DesignAI.

Go deeper

Exhaust fan calculator Waterproofing calculator Bathroom cost calculator

This is an indicative screening tool, not a substitute for a site survey. Real condensation depends on wall and glass temperatures, occupancy, drying habits and air-conditioning, which vary widely. Treat the risk score, recommended CMH and run-on time as a starting point and confirm fan sizing, ducting and waterproofing with your contractor.

Frequently asked questions

How does the bathroom humidity calculator work?: It estimates how much water vapour a hot shower adds to your bathroom air and whether ventilation clears it before it condenses. From your room volume, climate humidity, shower frequency and current ventilation it models the peak post-shower humidity and dew point, then flags whether cooler surfaces like mirrors and tiles will sweat and seed mould. It returns a risk score out of 100.
What inputs do I need to use it?: Enter the bathroom length, width and height in feet, pick a climate band that matches your city (coastal or humid, moderate inland, or dry), the number of showers per day, and your current ventilation, whether none, an openable window or an exhaust fan. If you have a fan, add its CMH rating. Tick the windowless option if the bathroom has no daylight or openable window.
How much exhaust ventilation does a bathroom need?: A common planning target is roughly eight to fifteen air changes per hour, so a small bathroom often needs an exhaust fan of about 80 to 120 CMH, more in humid coastal homes or windowless rooms. Run the fan through the shower and for the run-on minutes the tool suggests. These are indicative figures, so confirm fan sizing, ducting and layout on site and per NBC 2016 Part 8 ventilation guidance.

Related Guides — Deep-dive reading

Bathroom Condensation and Mould India: Stop Damp, Black Mould & Musty Smell for Good

Why warm, wet shower air keeps fogging your tiles, ceiling and walls with black mould — and the full Indian playbook to stop it: exhaust run-on, anti-fungal paint, wet-dry separation, wiping down, a dehumidifier, and safely cleaning the mould you already have.

Bathrooms

Exhaust Fan vs Natural Ventilation for Indian Bathrooms: Which Actually Keeps It Dry? (India)

A fair, room-by-room verdict on mechanical exhaust fans versus natural window-and-ventilator airflow — moisture and odour removal, the windowless reality, monsoon humidity, running cost, what NBC 2016 demands, and why the honest answer for most Indian bathrooms is 'both'.

Bathrooms

Bathroom Ventilation India: The Complete Guide to Moisture, Mould, NBC 2016 Air Changes & Windowless Solutions

Why bathroom ventilation matters more than any fitting in an Indian home — how moisture, mould and odour build up, natural versus mechanical options, what NBC 2016 actually requires, how to size a fan by air changes and CFM, and how to ventilate a windowless bathroom that stays dry through the monsoon.

Bathrooms

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