Last updated: January 2026
Need to know how much fluid your towel rail holds? This calculator gives you the internal volume (for filling with water/glycol) and recommends the correct element wattage. Essential for electric and dual fuel conversions.
🧮 Volume & Element Calculator
💡 Why Volume Matters
When converting a central heating towel rail to electric or dual fuel, you need to know the internal volume to:
- Size the element correctly — too small won't heat it, too large wastes energy
- Know how much fluid to add — underfill and it won't circulate, overfill and it may burst
- Add the right amount of inhibitor — protects against corrosion
📐 How We Size Your Element
We calculate the heat output based on surface area, then recommend an element 1.3x larger for efficient heating:
30-45 min
Heat-up time from cold
50-60°C
Surface temperature
Efficient
No wasted energy
Example: 10 litre rail × 30W = 300W element recommended
⚠️ Never Fill to 100%
Always fill to 87.5% capacity (some say 85-90%). The remaining space is an air cushion that allows the fluid to expand when heated. Without this, pressure builds up and can damage seals, blow the bung, or in extreme cases, split the rail.
📊 Quick Reference: Common Sizes
| Towel Rail Size | ~Volume | Fill Amount | Element |
|---|---|---|---|
| 400 × 800mm (12 bars) | ~4.5 litres | ~4.0 litres | 150W |
| 500 × 1200mm (20 bars) | ~8.5 litres | ~7.5 litres | 300W |
| 600 × 1200mm (22 bars) | ~10 litres | ~8.75 litres | 300W |
| 600 × 1600mm (28 bars) | ~14 litres | ~12 litres | 600W |
| 600 × 1800mm (32 bars) | ~17 litres | ~15 litres | 600W |
Values based on standard 22mm bar diameter. Use the calculator above for exact figures.
🛠️ What You'll Need for Electric Conversion
Inhibitor
Prevents corrosion
Shop Inhibitor →❓ Frequently Asked Questions
Can I use water or do I need special fluid?
You can use tap water with inhibitor added. For unheated spaces (risk of freezing), use a pre-mixed glycol solution instead.
What happens if I choose the wrong element size?
Too small: The rail won't get hot enough and will run constantly. Too large: It'll heat quickly but cycle on/off frequently, wasting energy. The 30W/litre rule gives the sweet spot.
Does this work for curved towel rails?
Yes. The curve adds minimal extra volume. Results are accurate within 5% for standard curved rails.
I have a designer radiator, not a ladder rail. Can I still use this?
This calculator is specifically for ladder-type towel rails. Designer/panel radiators have different internal structures. Check the manufacturer's specs or ask us for help.
📚 Related Calculators & Guides
Need Help With Your Electric Conversion?
We can recommend the exact element and accessories for your towel rail
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