Furnace sizing calculator
Floor area, ceiling height and a W per m³ ratio for your insulation and climate: you get the estimated heating load and the stock furnace or boiler size.
Recommended furnace size
12 kW
Net measurement: 8.19 kW
- Recommended furnace / boiler size12 kW
- Estimated heating load8.19 kW
- Kilocalories per hour7043.4 kcal/h
- Heated volume234 m³
Your prices per language/currency. Used when you enable “Use my prices” in calculators. My prices →
How it's calculated (visible formula)
kW = area × height × W/m³ ÷ 1000; pick the next stock size
- Net geometric measurement, no waste.
- Adjustable waste % for cuts and breakage.
- Purchase rounded up to sellable units (boxes, bags, buckets, boards).
How to calculate it, step by step
- Measure heated floor area.
- Multiply by ceiling height for volume.
- Pick W/m³ for insulation and climate.
- Work out heating output needed.
- Choose a furnace or boiler that covers it with good turndown.
Worked example: 970 sq ft home, 8.5 ft ceilings, 35 W/m³
- Volume: 234 m³ (8,260 ft³)
- 234 × 35 = 8,190 W ≈ 28,000 BTU/h
→ Choose a ~40,000 BTU/h (12 kW) furnace or the smallest modulating model.
Common mistakes
- Oversizing for hot water without checking modulation.
- Using the same ratio for a 1970s house and a new build.
- Ignoring infiltration in leaky homes.
Pro tips
- Size heat pumps at your local design temperature.
- For combi boilers, favor low minimum modulation.
- Balance radiators so every room heats evenly.
- Improve insulation before oversizing equipment.
From calculation to quote
Add labour, overhead and your margin, and generate the professional PDF.
FAQs
- How many BTU per square foot for heating?
- About 30–60 BTU/h per sq ft depending on climate and insulation.
- Why not just pick a big furnace?
- Oversized furnaces short-cycle, waste fuel and feel uneven.
- Need more precision?
- Use the heat load calculator with real U-values and areas, or a Manual J.
- What size furnace for 1,500 sq ft?
- Often 40,000–60,000 BTU/h output depending on climate.
- BTU per square foot for heating?
- About 30–60 BTU/h per sq ft.
- Radiant floor or radiators?
- Radiant floors run at low temperature, ideal with heat pumps.
Sources and standards
Maintained by Srikanta M · Formula v1.0.0 · Verified 2026-09-10 · Updated 2026-09-10 · Limits: quantity estimation, not structural design or site supervision.