Hot Tub Base Calculator – Is My Base Strong Enough?
Work out what your hot tub weighs when it is full of water and people, whether a standard deck or floor can take it, and exactly what a safe base needs.
Hot Tub Base Calculator
You need
476kg/m² load
2,100 kg filled, over a 4.41 m² footprint — about 4.67 kN/m²
- Filled weight
- 2,100 kg
- Load
- 4.67 kN/m²
- Base size
- 2.7 × 2.7 m
- MOT Type 1
- 1.53 t
How we worked this out
Is a standard deck or floor enough?
No. 476 kg/m² is well over the 150 kg/m² that ordinary domestic decking and floors are designed for — roughly 3.2× the usual allowance. Do not put this tub on standard decking without a structural engineer's design. Build it on ground instead.
Your shopping list
2 × 850kg bulk bags of MOT Type 1
1.53 tonnes at 100mm compacted.
£110
1 × 1200 gauge DPM sheet
Covers the 7.29 m² base with a turn-up all round.
£35.00
1 × m³ of C25 ready-mix concrete (rounded up)
You need 0.8 m³ — 100mm slab over 7.29 m², including 10% waste.
£550
3 × A142 mesh sheets (2.4 × 1.2m)
Laid on spacers in the middle third of the slab.
£96.00
11 × Metres of 100mm timber formwork
Shuttering, pegged and levelled before the pour.
£66.00
Typical hot tub weights and loads
| Tub | Dry weight | Water | Filled with bathers | Footprint | Load |
|---|---|---|---|---|---|
| Inflatable 4-person | 30 kg | 800 L | 1,150 kg | 1.8 × 1.8 m | 355 kg/m² |
| 2–3 seat | 250 kg | 900 L | 1,390 kg | 1.8 × 1.8 m | 429 kg/m² |
| 4–5 seat | 400 kg | 1,300 L | 2,100 kg | 2.1 × 2.1 m | 476 kg/m² |
| 6–7 seat | 500 kg | 1,800 L | 2,860 kg | 2.3 × 2.3 m | 541 kg/m² |
How this calculator works
A hot tub's advertised weight is the empty shell. What matters is the filled weight: dry weight, plus the water (one litre weighs one kilogram), plus the people in it at about 80kg each. Divide that by the footprint and you have the load in kilograms per square metre.
Structural engineers work in kilonewtons per square metre, so we show that too — multiply kg/m² by 0.00981. Ordinary domestic floors and decks are typically designed for about 1.5 kN/m², which is 150 kg/m². Nearly every filled hot tub is three or four times that, which is why "can I put it on the decking?" almost always has the answer "not as built".
For the base itself we take the tub footprint plus 300mm on each side, then work out 100mm of concrete over 100mm of compacted MOT Type 1, or paving slabs fully bedded on the same sub-base.
Worked example: a 4–5 seat tub
400kg dry, 1,300 litres of water and five bathers at 80kg gives 400 + 1,300 + 400 = 2,100kg. Over a 2.1m × 2.1m footprint (4.41m²) that is 476 kg/m², or 4.67 kN/m² — more than three times the usual domestic allowance.
The base for it is 2.7m × 2.7m, which is 7.29m². That needs about 1.53 tonnes of MOT Type 1 at 100mm and 0.8m³ of concrete at 100mm with waste — roughly £440 of materials as a ready-mix pour, plus mesh and shuttering.
Building a base that lasts
Dig out to 200mm below your finished level, lay and whacker the MOT Type 1 in two 50mm layers, then put down a DPM and shutter the edges dead level. Lay A142 mesh on spacers so it sits in the middle of the slab, pour C25 concrete, tamp it and float it off.
Give the slab at least seven days before the tub goes on it, and ideally 28 days before it is filled. Keep the surface just off level around the edge of the surrounding paving so rain runs away from the cabinet rather than pooling under it.
Think about access before you build: most tubs are craned or rolled in on their side and need a clear route at least as wide as the tub is deep. Leave service access on the panel side — usually the one with the control pack — and run the electrical supply in ducting under the slab while you have the chance.
Tips from the trade counter
- Never spot-bed slabs under a hot tub. Five dabs of mortar will rock and crack; use a full wet bed.
- Check the drainage. A 1,300 litre dump every few months has to go somewhere other than your neighbour's garden.
- Plan for the cover lifter. It needs clear space behind the tub.
- Get the deck checked, not guessed. If you are determined to use decking, a structural engineer's visit is a few hundred pounds well spent.
- Factor running costs in. A tub is a permanently heated 1,300 litre tank — use our appliance running cost calculator before you buy.
Electrical safety and permissions
A hard-wired hot tub needs a dedicated, RCD-protected circuit with an accessible isolator. That is notifiable work under Part P of the Building Regulations in England and Wales and must be carried out by a registered electrician, who will issue a certificate.
The tub itself rarely needs planning permission, but a permanent enclosure, gazebo or raised deck around it might, particularly in a conservation area or on a listed property. Check with your local council before building anything over it.
Frequently asked questions
How much does a hot tub weigh when full?
Dry weight plus the water (1 litre = 1kg) plus the bathers at about 80kg each. A four to five seat tub typically works out around 2,100kg once filled and occupied.
Can I put a hot tub on decking?
Only if the deck was designed for it. Ordinary domestic decking and floors are generally designed for about 150 kg/m² (1.5 kN/m²), and most filled hot tubs are well over 400 kg/m². Get a structural engineer to design or check the frame.
How thick should a hot tub base be?
A 100mm concrete slab on 100mm of compacted MOT Type 1 handles a domestic hot tub comfortably. Add A142 or A193 mesh for extra peace of mind on larger tubs.
How much bigger than the tub should the base be?
At least 300mm beyond the tub on every side. That gives somewhere to stand, keeps the cabinet off wet ground and leaves access for servicing.
Can I put a hot tub on paving slabs?
Yes, on a properly compacted MOT Type 1 base with the slabs fully bedded on mortar — not spot-bedded on five dabs, which will rock and crack under the load.
Does a hot tub base need to be level?
Yes, dead level in both directions. An out-of-level tub puts twist into the shell, spoils the water line and can void the warranty. Fall for drainage should be in the surrounding paving, not the base.
Do I need an electrician for a hot tub?
A plug-in 13A inflatable tub can go on an outdoor RCD socket. A hard-wired tub needs a dedicated, RCD-protected circuit and an isolator — notifiable work under Part P, so use a registered electrician.