What Size Space Heater Do I Need? Wattage To Sq Ft Chart + Calculator

Space heater coverage area. This is not all that easy to figure out. We will help to check how many square foot can different wattage space heaters cover. Namely, we see two kinds of questions here:

  1. “How much space can a 1500 watt heater heat?” Here we already have the space heater size (1500 watt) and want to figure out how many square feet can such a heater heat.
  2. “How many watt heater do I need to heat 500 sq ft?” Here we have a space we want to heat and want to figure out how many watt space heater do we need.
what size space heater do i need watts to sq ft conversion
Basic watts to sq ft and sq ft to watts conversion we will be using based on 30 BTU per sq ft rule of thumb for adequate heating.

We will show you how to answer both of these questions for any wattage heater and any space size.

The basic rule of thumb in HVAC is that you need 30 BTU of heating output per sq ft of living space. Example: A 100 sq ft room requires at least 3000 BTU of heating output.

Here is the problem:

When we are looking at electric heaters, we don’t have 1000 BTU, 2000 BTU, 3000 BTU space heaters. Instead, we have 500W, 1000W, 1500W, 2000W heaters and so on.

In order to figure out what size space heater you need, we need to convert these wattages into BTUs, and then apply a 30 BTU per sq ft rule of thumb.

Example: How many square feet can a 1500 watt heater heat? A 1500 watt heater can produce 5118 BTU of heating output. Applying the 30 BTU per sq ft rule, we see that a 1500W heater can heat up to 171 sq ft.

We will show you how to calculate the size of the heater you need for any space. We will also look at how much space can different space heaters heat.

Note: For supplemental heating (furnace in addition to space heater), you can will need about 10 BTU per sq ft. That means that a 1500W heater with 5118 BTU output can provide supplemental heating to 512 sq ft.

Here is how we are going to illustrate everything you need to know about space heater sizing in 3 useful resources:

  • How to calculate the coverage space of space heaters, and how to calculate the space heater wattage needed to cover the certain square footage.
  • 2 Space Heater Size Calculators. In the 1st calculator, you insert the wattage (Example: 1500W) and the calculator will tell you how much space such a space heater can heat. In the 2nd calculator, you insert square footage (Example: 500 sq ft) and the calculator will tell you how many watt heater you need.
  • Space Heater Coverage Chart. A complete for how much square footage can 100W to 20,000W heaters heat + supplemental heating square footage.

Let’s start by illustrating how these space heater sizing calculations are made:

What Size Space Heater Do You Need? (Manual Calculation)

We want to figure out how many watt electric heater do we need. The main difficulty here is to convert heater wattage to BTUs. Namely, we need 30 BTU of heating output to heat 1 square foot of living area.

We know that 1 watt is equal to 3.412 BTU. Based on this, we can calculate how many watts do we need to heat 1 sq ft of living space like this:

Watts To Heat 1 Sq Ft = (30 BTU / sq ft) / 3.412 BTU = 8.79 Watts

In short, we need 8.79 watts to heat 1 sq ft of living space. Roughly, you can use a 10 watts per sq ft approximation as well.

With this in mind, we can easily calculate:

  • How much space can a 1500 watt heater heat.
  • What size heater do we need for 500 sq ft.

Namely, if we have a 1500 watt heater, we calculate the coverage area like this:

Coverage Area (1500W Heater) = 1500 Watts / (8.79 Watts / Sq Ft) = 171 Sq Ft

This means that a 1500 watt heater can heat up to 171 sq ft of living space. Now, we can calculate how many watts do we need to heat a 500 sq ft space, for example, like this:

Heater Size For 500 Sq Ft = 500 Sq Ft × 8.79 Watts / Sq Ft = 4395 Watts

To heat up 500 sq ft, we need 4395 watt heater. That is about 3 standard 1500W heaters with the combined wattage of 4500 watts.

These calculations can be a bit complex. That’s why we designed these two calculators that do these calculations automatically:

Heater Coverage Calculator (1st Calculator)

Here you just insert the wattage of your heater and the calculator will tell you what space it will be able to heat adequately:

 

Example of using the 1st calculator: How much space can a 2000 watt heater heat? Just slide the slider to ‘2000’ and you get the result: A 2000 watt heater can heat 227 sq ft of space.

Wattage Needed To Heat A Certain Space Calculator (2nd Calculator)

Here you insert the square footage and the calculator will tell you how many watt heater you need to heat it:

 

Example of using the 2nd calculator: What size heater do I need to heat up a 300 sq ft room? Just slide the slider to ‘300’ and you get the result: To heat up a 300 sq ft room, you need a 2637 watt heater (practically, that’s two 1500W heaters).

Space Heater Coverage Chart

This chart tells you how many sq ft can different-size heaters heat:

Heater Wattage: Coverage (30 BTU/Sq Ft):
100 Watts 11.4 Sq Ft
200 Watts 22.7 Sq Ft
300 Watts 34.1 Sq Ft
400 Watts 45.5 Sq Ft
500 Watts 56.8 Sq Ft
600 Watts 68.2 Sq Ft
700 Watts 79.6 Sq Ft
800 Watts 90.9 Sq Ft
900 Watts 102 Sq Ft
1000 Watts 114 Sq Ft
1250 Watts 142 Sq Ft
1500 Watts 171 Sq Ft
2000 Watts 227 Sq Ft
3000 Watts 341 Sq Ft
4000 Watts 455 Sq Ft
5000 Watts 568 Sq Ft
10,000 Watts 1140 Sq Ft
15,000 Watts 1710 Sq Ft
20,000 Watts 2270 Sq Ft

With this chart and both calculators, you can now quite easily determine what size electric space heater you need. If you have questions about heater sizing, you can use the comments below and we’ll try to help you out.

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