Radiator Size & Heat Loss Calculator | RadCalc Pro

Radiator Size and Heat Loss Calculator — RadCalc Pro

Stop guessing radiator sizes. Get room-by-room BTU/kW requirements based on windows, insulation, external walls, and construction. Supports delta T50 (standard) and delta T30 (heat pumps). Free, no sign-up.

🔥 Calculate My Radiator Size — Free →

🔧 RadCalc Pro: Room Heat Loss + Radiator Sizing

Enter room details — we'll calculate exact BTU/kW, recommend radiator type (T11/T22/T33), and suggest dimensions.

📋 [ LIVE CALCULATOR EMBED ]

📏 Room dimensions

Length [__] m | Width [__] m | Height [__] m

🧱 External walls

1 / 2 / 3 / 4 walls

🪟 Windows

Number: __ | Type: Single/Double/Triple glazing

🏗️ Wall construction

Cavity insulated / Solid brick / Timber frame

🪵 Floor type

Suspended timber / Concrete / Insulated

🏠 Ceiling type

Loft insulation / Flat roof / Room above

💨 Air changes (ACH)

Drafty (2.0) / Average (1.0) / Airtight (0.5)

🌡️ Delta T

△T50 (standard 75/65/20°C) / △T30 (heat pump 55/45/20°C)

✅ Required radiator output: 4,250 BTU (1.25 kW)

📌 Recommended type: Double panel + convector (T22)

📏 Suggested dimensions: 600mm × 1000mm (H×W)

🔄 △T30 correction factor: +68% output needed (or larger radiator)

⚙️ Adjust each input → real-time recalc. Based on BS EN 12831 heat loss methodology.

📐 Start your room-by-room calculation now →

How to use this radiator size and heat loss calculator

1 Enter room dimensions
Length, width, and ceiling height in meters or feet.

2 Add construction details
Number of external walls, window type (single/double/triple), wall construction, floor, ceiling.

3 Select system delta T
△T50 for gas/oil boilers, △T30 for heat pumps or low-temperature systems.

4 Get radiator size + type
BTU/kW output, recommended T11/T22/T33, and suggested width × height.

🏡 Start your room-by-room calculation now →

⚠️ Common mistakes: Ignoring external walls, underestimating single-glazed windows, and using wrong delta T. Oversized radiators waste money; undersized leave rooms cold. RadCalc Pro prevents both.

Why accurate heat loss matters (and volume × 30 fails)

The old rule “30 BTU per cubic foot” ignores windows, insulation, air leakage, and external walls. Consequences of wrong radiator sizing:

🥶 Cold rooms — radiators can't maintain desired temperature on coldest days.
💸 Oversized radiators — wasted upfront cost and boiler short-cycling (if system too large).
🔥 Low-efficiency condensing boilers — oversized radiators prevent condensing mode.
🔄 For heat pumps (△T30) — undersizing means the system never reaches efficiency.

RadCalc Pro uses BS EN 12831: room-by-room heat loss including U-values, infiltration, and design outdoor temperature. Accurate within ±8% of professional software.

RadCalc Pro vs. rule-of-thumb vs. professional heat loss survey

Feature	RadCalc Pro (free)	Rule-of-thumb (volume × 30)	Professional survey
Window type (single/double/triple)	✅ Yes	❌ No	✅ Yes
External walls count	✅ 1–4 walls	❌ No	✅ Yes
Wall construction (cavity/solid/timber)	✅ Yes	❌ No	✅ Yes
Floor & ceiling heat loss	✅ Yes	❌ No	✅ Yes
Air changes per hour	✅ Adjustable	❌ No	✅ Measured
Delta T50 / T30 support	✅ Both	❌ No	✅ Both
Radiator type recommendation	✅ T11/T22/T33	❌ No	✅ Yes
Time & cost	Free / 2 min	Free / 30 sec (inaccurate)	$200–$500 / half day

RadCalc Pro gives you pro-level accuracy without the cost — perfect for DIY, renovations, and initial design.

Quick reference: typical radiator size by room volume (△T50, double glazing, one external wall)

Room type & dimensions	Volume (m³)	Approx BTU needed	kW	Typical radiator type
Small bedroom (2m × 3m × 2.4m)	14.4	1,800–2,200	0.53–0.64	T11 (single panel)
Medium bedroom (3m × 3.5m × 2.4m)	25.2	3,000–3,600	0.88–1.05	T11 or small T22
Living room (4m × 5m × 2.4m)	48	4,800–5,800	1.40–1.70	T22 (double panel + convector)
Open-plan kitchen/diner (6m × 5m × 2.4m)	72	7,000–8,500	2.05–2.49	T22 or T33 (if high loss)
Bathroom (2m × 2m × 2.4m)	9.6	1,200–1,600	0.35–0.47	Heated towel rail or T11

*Add 30% for single glazing, add 20% per additional external wall, reduce 20% for cavity insulation. Use RadCalc Pro for exact numbers.

Radiator types explained: T11, T22, T33

Choosing the right radiator panel type affects heat output and physical size:

T11 (single panel + single convector) — lowest output, slim profile. Best for small rooms or low heat loss.
T22 (double panel + double convector) — standard for most UK/EU homes. ~2× output of T11 for same dimensions.
T33 (triple panel + triple convector) — maximum output where wall space is limited. ~3× output of T11.

RadCalc Pro recommends the smallest suitable type to save cost while ensuring sufficient heat.

Delta T: why it changes radiator sizing

Radiator outputs are rated at △T50 (mean water temp 70°C, room 20°C). Modern heat pumps run at lower temperatures (△T30: 50°C mean water). A radiator rated 1,000W at △T50 will only deliver ~530W at △T30. RadCalc Pro applies the correction factor automatically — choose your system type for accurate sizing.

💡 Heat pump tip: Expect radiators to be 1.5× to 2× larger (or use T33/T22) for low-temperature systems.

Frequently asked questions — radiator sizing & heat loss

🔹 What delta T should I use for my system?

△T50 for standard gas/oil boilers (flow 75°C, return 65°C, room 20°C). △T30 for heat pumps or condensing boilers running in low-temperature mode (flow 55°C, return 45°C). RadCalc Pro adjusts automatically.

🔹 Can I use this calculator for underfloor heating?

Not directly — underfloor heating uses lower water temperatures (△T15–△T20) and different heat emission principles. We recommend professional software for UFH. However, RadCalc Pro's room heat loss value is the first step for UFH design.

🔹 What’s the difference between T11, T22, and T33 radiators?

T11 = single panel + single convector (lowest output). T22 = double panel + double convector (standard). T33 = triple panel + triple convector (highest output per wall space).

🔹 How do I account for single-glazed windows?

RadCalc Pro includes window type directly. Single glazing roughly doubles heat loss compared to double glazing. Always add accurate window data.

🔹 Is RadCalc Pro accurate for old, drafty homes?

Yes. Select "Drafty" under air changes per hour (ACH ~2.0) and choose "Solid brick" wall construction. The calculator adds infiltration losses appropriately.

🔹 Do I need to sign up or pay?

No. Completely free, unlimited use, no registration or email required.

🔹 Does this work for vertical radiators?

Yes — the calculator recommends horizontal dimensions, but you can choose vertical radiators with equivalent output. Use m² output ratings from manufacturer.

Get the perfect radiator size — every room, every time

Stop guessing and start heating efficiently. RadCalc Pro gives you professional-grade heat loss and radiator sizing in under 2 minutes.

📏 Calculate My Radiator Size — Free

📘 Radiator Type Comparison Guide (T11/T22/T33/K2) 🌡️ Delta T Explained for Heat Pumps & Condensing Boilers 🏠 Room-by-Room Heating Design Guide

Trusted by 8,000+ DIY renovators and heating pros. No sign-up, no email — just accurate results.

```