What this checker works out
This checker divides an item's weight by its footprint to give a distributed load in kilograms per square metre, and compares that against the design figure for the type of floor.
It is a rough comparison, not a structural assessment. Real floor capacity depends on joist size, span, spacing, condition, and above all where on the span the load sits. Anything heavy, concentrated or permanent needs a structural engineer rather than a calculator.
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Before you rely on the comparison
- Use the loaded weight. An aquarium's water is roughly a kilogram per litre, and a full 200 litre tank weighs well over 200 kg with substrate and glass.
- Footprint means the area actually in contact with the floor. Four small feet concentrate load far more than a solid base of the same overall size.
- Upper floors in dwellings are designed for about 1.5 kN/m², roughly 153 kg/m². Ground floors on a solid slab are much less sensitive.
- A suspended timber ground floor behaves like an upper floor, not like a slab.
A load in the middle of a joist span produces roughly four times the bending moment of the same load near a supporting wall. Placing a heavy item against a wall that runs at right angles to the joists, ideally over or close to a support, transforms the situation without changing the weight at all.
How this calculator works
A distributed load compared against a design figure:
Distributed load = weight ÷ (footprint length × footprint width)
Guide figures: upper floor about 153 kg/m² · ground floor slab about 250 kg/m²The 1.5 kN/m² figure is the imposed load that domestic floors are designed for under the structural Eurocodes and Approved Document A. It is a uniformly distributed load across the whole floor, which is not the same thing as a concentrated load in one place — which is precisely why exceeding it in a small footprint is not automatically a failure, and staying under it is not automatically safe.
The checker does not know your joist sizes, spans, spacing or condition, and cannot substitute for someone who does.
Worked example: 300 kg on a small footprint
Using the default figures — 300 kg over a 1.2 m by 0.5 m footprint, upstairs:
- Footprint area: 0.6 m²
- Distributed load: 500 kg/m²
- Guide figure for an upper floor: about 153 kg/m²
- Result: above the guide figure, by more than three times
That is roughly a large aquarium, a substantial safe or a cast iron bath full of water. Three times the design load in a concentrated footprint is exactly the situation where a floor may well be fine and may well not, depending entirely on where it sits relative to the joists and what condition they are in. This is the point at which the correct next step is an engineer, not a heavier tick from a website.
Common mistakes
- Using the empty weight. Water, books and contents dominate the total.
- Treating a result under the guide as approval. Joist condition and span position matter more.
- Placing heavy items mid-span. Against a supporting wall is far better.
- Assuming a ground floor is always fine. A suspended timber ground floor is not a slab.
- Skipping professional advice on a permanent heavy load. A structural engineer's fee is small against the alternative.
Frequently asked questions
Can my floor take an aquarium?
It depends far more on placement than on the total. Water alone is about 1 kg per litre, so a 200 litre tank with substrate, glass and stand can exceed 250 kg over a small footprint. Position it against a wall running at right angles to the joists, spread the load with a solid base, and for anything above about 150 litres upstairs, get it checked.
How do I find which way my joists run?
They usually run at right angles to the longest wall, and across the shortest span. A stud detector finds them through the floor, floorboard nail lines show their position, and from below in a cellar or through a loft hatch they are directly visible. Knowing the direction tells you which walls are useful for placing weight against.
What is 1.5 kN/m² in practical terms?
About 153 kg per square metre spread evenly. For comparison, a crowd of people standing shoulder to shoulder is roughly that. Domestic floors carry it comfortably with a safety margin, but the figure assumes distribution — the same total concentrated on four small feet in the middle of a span is a different problem.
Do I need building regulations approval?
Placing furniture does not require approval. Altering the structure to carry more — adding support, doubling joists, installing a beam — is structural work and engages building regulations. Converting a loft or removing a wall certainly does. If in doubt, your local authority building control team will tell you.
Is what I enter stored?
No. Weights and dimensions are processed in your browser and never transmitted or retained.
Related tools
References
- GOV.UK — Approved Document A on structure and domestic floor loading
- Health and Safety Executive — manual handling of heavy items and safe working practice
Sources are checked at publication and can change — how I choose and check references.