Boiler Size Calculator: Find the Right kW Rating
Boiler sizing comes down to two loads. One is the heat a house loses through walls, roof, windows and doors. The other is hot water demand from taps and showers. A combi handles both from the same box, which means the domestic hot water figure usually sets the floor. A system or regular boiler splits them across a cylinder and a heating circuit.
This boiler size calculator works through both. Enter the property’s dimensions, the insulation and glazing, the number of bathrooms, and the climate zone, and it returns the required kW output, the BTU equivalent, and a recommended size from standard boiler ratings. Combi, system, and regular layouts each get their own DHW treatment.
If you’re replacing an old unit or sizing one for a renovation, the number you need is the required kW, not the figure on the old boiler’s badge. Old boilers were often oversized on purpose.
How to Use the Boiler Size Calculator
Pick your currency from the top dropdown.
Enter the property length, width, and wall height, then set the unit for each.
Pick the insulation level that matches the building. Old uninsulated homes need the lowest setting.
Set the glazing type: triple, double, or single.
Count the windows and external doors.
Add the number of occupants and bathrooms.
Choose the climate zone and boiler type. Combi, system, and regular each carry a different hot water load.
Set the safety margin, then enter your price per kW and any additional cost.
The Summary tab leads with required output in kW. Detailed Breakdown lists every multiplier and adjustment behind the result.
How the Boiler Size Calculator Formula Works
The calculation stacks heating load and hot water load, then applies a safety margin. The final number snaps to a standard boiler rating.
Formula: Base heat loss = Floor area (m²) × 60 W/m²
Formula: Adjusted heat loss = (Base × Insulation × Glazing × Climate) + (Windows × 150 W) + (Doors × 200 W)
Formula: DHW load = 24 kW + 3 kW per extra bathroom (combi), or 3 kW per bathroom (system / regular)
Formula: Required output = (Heating kW + DHW kW) × (1 + Safety %)
Formula: BTU/hr = Required output in kW × 3,412.14
The 60 W/m² figure is a standard UK rule for average insulation. It converts to roughly 5.6 W/ft². The multiplier stack then shifts that base up or down. Excellent insulation drops it to 70 percent. Very poor insulation lifts it to 160 percent. Glazing adds another 0.9× to 1.15×. Climate swings wider, from 0.85× in mild zones to 1.4× in severe cold.
Each window adds a flat 150 W. Each external door adds 200 W. Those adders capture air leakage and conduction paths that don’t scale with wall area alone.
The DHW load is where most people get surprised. A combi heats water instantly, so it needs a minimum output near 24 kW just to serve one shower at a normal flow rate. Each extra bathroom pushes that up by 3 kW. A system or regular boiler stores hot water in a cylinder, so its DHW load is only 3 kW per bathroom.
After the totals, the safety margin inflates the result. The calculator defaults to 15 percent. Ten percent is often enough for residential work.
The recommended size snaps to standard kW ratings: 12, 15, 18, 21, 24, 26, 28, 30, 32, 35, 38, 40, 45, 50, or 60. Above 60 kW, it rounds up to the nearest multiple of 5.
Worked Example: A 28 ft × 22 ft Three-Bedroom House
Say you’re sizing a combi boiler for a small three-bedroom house. The footprint is 28 ft × 22 ft, ceilings are 8 ft. Insulation is excellent after a recent renovation. Triple glazing. Eight windows, one external door, three occupants, one bathroom. Climate zone is mild. Safety margin is 10%. Installed price is $180 per kW.
Floor area: 28 × 22 = 616 ft² (57.23 m²)
Total volume: 616 × 8 = 4,928 ft³
Base heat loss: 57.23 × 60 = 3,434 W
Insulation (excellent): × 0.7
Glazing (triple): × 0.9
Climate (mild): × 0.85
Adjusted base: 3,434 × 0.7 × 0.9 × 0.85 = 1,839 W
Windows: 8 × 150 = 1,200 W
Doors: 1 × 200 = 200 W
Total adjusted heat loss: 3,239 W, or 3.24 kW
DHW load (combi, one bathroom): 24 kW
Total heat demand: 3.24 + 24 = 27.24 kW
With 10% safety: 27.24 × 1.10 = 29.96 kW
BTU/hr: 29.96 × 3,412.14 = 102,238 BTU/hr
Recommended size: 30 kW
Equipment cost: 30 × $180 = $5,400
The heating load is tiny, barely over 3 kW, because the house is well sealed. The hot water requirement drives the choice. A combi has to heat mains water to shower temperature the moment the tap opens, and that minimum output doesn’t shrink with the house. Small homes and large ones often land on similar combi sizes for exactly this reason.
