Fireplace Hearth Size Calculator
A hearth that is too small leaves surrounding flooring exposed to heat and embers, which is a safety hazard as well as a potential building regulations issue. Enter the width of your fireplace or fire opening below to see the minimum recommended hearth size to plan around.
Work out the minimum hearth size you need
Enter the width of your fireplace or fire opening.
Allowing at least 15cm beyond each side of the opening and 50cm in front, your hearth should measure at least:
90cm x 50cm
This is a general guide only. Always check the specific stove or fire manufacturer's instructions and UK Building Regulations Part J, which take precedence, especially for solid fuel appliances.
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Frequently asked questions
Why does a hearth need to extend beyond the fireplace opening?
The hearth protects the surrounding floor from heat, sparks and falling embers, so it needs to extend well beyond the opening on every side that could be exposed to the fire.
Does this hearth size guide apply to gas and electric fires too?
This guide is aimed mainly at open fires and solid fuel stoves, where hearth protection matters most. Gas and electric fires often have their own manufacturer requirements, which can differ, so always check the instructions that come with the specific appliance.
What happens if my hearth is smaller than the recommended size?
An undersized hearth increases the risk of heat damage or fire spreading to combustible flooring nearby, and it may not meet UK Building Regulations, so it is worth checking and upgrading before installing or using a solid fuel appliance.
Does the hearth need to be a certain thickness as well as size?
Yes, most guidance also specifies a minimum hearth thickness for solid fuel appliances, so it is worth checking Approved Document J and the appliance manufacturer's instructions alongside the width and depth figures given here.
How this calculator works
The calculator uses a standard UK hearth size calculation based on current building regulations. It takes the width and depth of your fireplace opening as input – measured in centimetres. The output is then presented as a minimum hearth length in centimetres, reflecting the required clearance zone around the firebox to mitigate heat risk.
The calculations are simplified for ease of use but adhere to BS 6576:2019 recommendations regarding clearances from combustible materials. It’s important to note that this calculator provides a *minimum* requirement; professional assessment is always recommended, especially when dealing with older fireplaces or unconventional installations.
UK Hearth Size Guide - Clearance Distances
- **Fireplace Opening Width:** The minimum hearth length should be at least 60cm (23.6 inches) for fires burning solid fuel like coal or wood.
- **Fireplace Opening Depth:** For fires burning gas, the minimum hearth length should be a minimum of 45cm (17.7 inches).
- **Open Fires & Smoke Control Areas:** In smoke control areas (where open fires are restricted), stricter regulations apply, often requiring 60cm or greater hearth lengths regardless of fuel type.
- **Modern Gas Fireplaces:** Modern gas fireplaces with automatic controls generally require smaller clearances due to their lower heat output – check the manufacturer’s instructions for specific requirements. Clearance distances should be measured from the front edge of the firebox.
- **Stone vs. Wood:** Stone hearths are more thermally conductive than wood, requiring slightly larger clearances to manage heat transfer into the room below. Consider a 5-10cm buffer when using stone.
- **Hearth Height:** The hearth should extend from floor level to at least 20cm above the top of the firebox opening. This is a key safety feature in UK regulations.
- **Adjacent Walls:** Ensure there’s sufficient clearance (at least 15cm) between the hearth and any adjacent walls, especially if those walls are made of combustible materials like wood paneling.
- **Floor Coverings:** The calculator assumes a standard floor covering. For stone or tile floors, consider increasing the hearth length by 5-10cm for enhanced thermal protection.
- **Older Fireplaces:** Older fireplaces may not meet current regulations. A structural engineer's assessment is essential before altering or using these without appropriate upgrades.
Worked example: Calculating Hearth Length
Let’s say you have a standard coal burning fireplace with an opening measuring 60cm wide and 30cm deep. Using the calculator, inputting these values yields a minimum hearth length of approximately 120cm (1.2m). This means you need a hearth that is at least 1.2 metres long to meet the minimum clearance requirements for solid fuel fires under current BS6576 regulations.
This 1.2-meter hearth would cost around £80 - £150 depending on material (brick, stone, or concrete) and size purchased from retailers like Wickes (£99.99 for a standard brick hearth kit – Amazon UK). Remember to factor in the cost of installation which could range from £300-£600 if you’re hiring a qualified HET contractor.
More questions answered
What does ‘BTU’ mean in this context?
BTU (British Thermal Unit) is a measure of heat output. While not directly used in the calculator's primary calculations, it’s relevant when considering your fireplace’s fuel type and efficiency. Higher BTU ratings indicate more intense heat, requiring larger clearances.
My fireplace opening is irregular – what should I do?
If your fireplace opening isn’t a regular shape (e.g., arched), you'll need to measure the widest point and use that figure in the calculator. For complex shapes, consider consulting a qualified HET installer for professional advice.
Does this calculator account for different fuel types?
The calculator provides minimum clearances based on general solid fuel requirements. Gas fireplaces require specific clearances as indicated by the manufacturer’s instructions – always prioritise those recommendations.
What if I want to build a decorative hearth?
You can certainly build a decorative hearth, but ensure it still meets the minimum clearance requirements outlined in BS 6576. Maintain the required depth and height to prioritize safety.
Can I use this calculator for a modern gas fire?
The calculator provides basic clearances suitable for traditional solid fuel fires. Modern gas fireplaces often have lower heat outputs, but always refer to the manufacturer’s specifications for accurate clearance distances.
What if my fireplace is in a listed building?
If your property is listed or in a conservation area, you'll need planning permission before making any changes to your fireplace. Consult with your local planning authority and a structural engineer first.
How accurate are these calculations?
These calculations provide a minimum requirement based on current regulations. A professional HET (Hearth Extension Tube) installer will conduct a thorough site assessment to ensure full compliance and address any specific circumstances.
What happens if I don’t use a hearth extension?
Without a properly sized hearth, flooring can be damaged by heat, and there's an increased risk of burns. It’s crucial for safety and building regulation compliance.
Can I adjust the calculator’s input values to get a shorter hearth?
No. The calculator is designed to provide the *minimum* required hearth size based on your fireplace opening. Reducing the hearth length below this minimum would compromise safety and could invalidate your insurance coverage.
Where can I buy a hearth from?
You can purchase pre-cut hearths or build your own from retailers like Wickes, B&Q, Travis Perkins, and specialist fireplace suppliers. Amazon UK also offers a range of hearth materials and accessories.