Bathroom electric heaters are sized at roughly 10 watts per square foot of floor area, so a 40-square-foot bath needs about 400 watts minimum — though 750 to 1,500 watts warms it far faster.
Buy the wrong wattage and you get one of two expensive outcomes: a heater that runs constantly and never takes the chill off, or a circuit that trips every time the bathroom door closes. The math behind getting it right takes about two minutes, and it starts with a tape measure.
Here’s the short version. Measure the floor, multiply the square footage by 10, then read the actual product page before you buy — because a unit’s listed coverage area, voltage, and amperage decide whether it works in your bathroom, not the box art.
How Many Watts Does Your Bathroom Actually Need?
Multiply your bathroom’s floor area by 10 watts per square foot for a baseline, then size up if the room is poorly insulated, has a high ceiling, or sits in a cold climate. Sizing guides from Sylvane recommend that starting point for average insulation, and it lands in a sensible range for most U.S. bathrooms.
To measure, multiply length by width in feet. A 5 x 8 ft bathroom is 40 sq. ft., which puts the bare minimum at 400 watts — but most people find that a 750- to 1,500-watt unit heats the room much faster and holds the temperature better on a cold morning.
If you’d rather think in BTU, divide your wattage by 3.41 to convert watts to BTU. Keep in mind that heater sizing runs on watts or BTU, never on CFM — CFM describes airflow for vent fans, a completely different job.
Bathroom Size to Heater Wattage: A Quick Reference
| Bathroom Size | Minimum Watts | Recommended Watts |
|---|---|---|
| 15–25 sq. ft. (half bath) | 250 W | 500–750 W |
| 35–50 sq. ft. (standard full bath) | 400 W | 1,000–1,500 W |
| 60–100 sq. ft. (large master bath) | 600 W | 1,500–2,000 W |
| 40 sq. ft. worked example | 400 W | 750–1,500 W |
| Broan 162M (official spec) | 250 W rated, up to 65 sq. ft. | 120 V, 2.5 A |
| Broan 157 (official spec) | 1,250 W, covers 85–125 sq. ft. | 120 VAC, 10.7 A |
| Broan 659 fan-heater combo | 1,440 W, bathrooms up to 45 sq. ft. | 120 V |
The minimum column is exactly that — a floor, not a target. If your bathroom sits on an exterior corner, has a 10-foot ceiling, or never quite warms up in January, treat the recommended column as your real starting point.
What the Product Page Tells You That the Rule of Thumb Can’t
A wattage rule gets you close, but the product page decides whether a specific heater fits your room, your wiring, and your ceiling. Broan’s official listings for its heater models spell out coverage area, voltage, and amperage — and those three numbers can disqualify a unit the square-footage math liked. Comparing several models side by side makes this much faster, and our tested roundup of the best bath wall heater picks puts the key specs in one place so you can match a unit to your room before you buy.
Check these four things on every product page:
- Listed coverage area. Broan rates model 162M for up to 65 sq. ft. at 250 watts, 120 volts, and 2.5 amps. Broan’s model 157 is a fan-forced ceiling heater rated 1,250 W at 120 VAC that covers 85–125 sq. ft. Both are real coverage claims — trust them over your own estimate.
- Voltage and amperage. A 1,250-watt heater pulling 10.7 amps needs a circuit that can carry it alongside the lights and outlet on that line.
- Installation type. Broan’s 157 is a ceiling heater, not a wall unit. Where it mounts changes both the install and the coverage pattern.
- Wet-location listing. If your heater goes above the tub, this line is the one that matters most.
One more wrinkle: Broan publishes different spec pages for the U.S. and Canadian markets, so confirm you’re reading the listing for your region before you commit to a model number.
The Mistakes That Waste Money
Most bad bathroom heater purchases trace back to skipping the product page. The square footage matched, so the box went home — and the unit turned out to be a 250-watt model rated for a closet-sized room, or a heater that draws more amps than the bathroom circuit can spare. Broan’s own spec sheets exist precisely to prevent this, and they take thirty seconds to read.
Two smaller traps: sizing with CFM rules borrowed from vent fans (wrong unit of measure entirely), and ignoring ceiling height or insulation quality, both of which can push your real wattage need well above the 10 W/sq. ft. baseline. When in doubt, go up one step in wattage rather than down — an oversized heater cycles off sooner and costs less to run than an undersized one that never stops.
If the math puts you between two sizes, the higher-wattage unit is almost always the better buy for a bathroom that gets daily morning use.
FAQs
Is 1,500 watts too much for a small bathroom?
Not necessarily. A 1,500-watt unit in a 40-square-foot bath heats the room quickly and then cycles on and off, which uses less energy than a small heater running nonstop. The real limit is your circuit: check the amperage on the product page against what the bathroom line can safely carry.
Can I put an electric heater directly above the shower?
Only if the unit is listed for that location. Check the wet-location rating on any heater before mounting it above or beside the tub.
Do high ceilings really change the wattage I need?
Yes. The 10 watts per square foot rule assumes an average ceiling height with decent insulation. A bathroom with a 10-foot ceiling or an exterior corner wall holds more cold air, so step up to the recommended wattage range for your square footage rather than the minimum.
References & Sources
- Sylvane. “How to Select a Bathroom Heater” Supports the 10 W/sq. ft. sizing rule, the watts-to-BTU conversion, and the CFM caution.
- Broan-NuTone. “157 Fan-Forced Ceiling Heater” Official coverage, wattage, and amperage specs for model 157.
- Broan-NuTone. “162M Ventilation Fan” Official specs and the GFCI/shower-location limitation for model 162M.
