What this calculator works out
A shed needs something solid, level and dry underneath it, and there's more than one legitimate way to build that. A concrete slab is the most permanent option; paving slabs and a plastic grid system both build a base from smaller, liftable units; a timber frame on bearers is the lightest option and the one most sheds arrive expecting if they come with their own floor kit. This calculator takes the shed's own footprint, adds whatever border you want around it, and then works out the full materials list for whichever base type you're actually building.
It cross-links this site's Concrete Calculator and Concrete Bags Calculator for turning a concrete volume into a ready-mix order or a bag count, and reuses the Paving Calculator's slab-count method and the Hardcore Calculator's and Gravel Calculator's tonnage conventions, so the figures line up with the rest of the site rather than starting from scratch.
How big should the base be?
Every base type starts from the shed's own footprint, but the base itself is rarely built to that exact size, and guides genuinely disagree on why. Most recommend a border: ConcreteMath's shed base guide suggests 50–100mm larger on all sides so rain off the roof lands on the base rather than the ground beside it, and Greenhouse Stores' concrete shed base guide goes further, recommending 100–150mm "to give you room for drainage and prevents water pooling against the timber frame". A wider base also gives a working margin for squaring up and levelling.
The opposing case is narrower but specific to concrete: SitePrep's comparison of gravel and concrete shed foundations argues a concrete base should stay the same size as the shed, not wider, because the exposed strip a wider base leaves next to the wall becomes a shelf where rainwater sits and can cause "moisture damage to the building frame", rather than draining away as a gravel bed would. Neither answer is wrong; this calculator offers same-size, +50mm and +150mm options rather than picking a side, since the right choice depends on the ground, the base type and how exposed the site is.
Concrete slab
The most permanent of the four, built in the same two layers this site's Paving Calculator and Hardcore Calculator use: compacted MOT Type 1 first, then the slab poured on top. Depth scales with the shed's size, following ConcreteMath's shed base guide: around 75–100mm of slab on 75–100mm of hardcore for a small shed, up to 150mm on 100mm for a workshop. Between the two goes a damp-proof membrane, typically 1200-gauge polythene, edges turned up against the formwork "to form a tray" and lapped by at least 300mm at any joint, which is why this calculator adds a 10% allowance for the membrane figure rather than the bare footprint.
Greenhouse Stores' guide also gives a mix for the slab itself, a C20-equivalent 1:2:3 (cement:sand:aggregate) mix by volume for a domestic shed base. Once the concrete volume is known, this site's Concrete Calculator turns it into a bag-mix or ready-mix order, and the Concrete Bags Calculator works out exactly how many pre-mixed bags that volume needs.
Paving slabs
A liftable, no-mixing alternative built the same way this site's Paving Calculator works out a patio: slabs sized by their footprint plus a share of the joint around them, on a compacted MOT Type 1 sub-base and a sharp sand bed. A shed base typically uses a tighter joint than a patio, since it's walked on rather than looked at, and this calculator defaults to 5mm rather than the 10mm a patio commonly uses. The sub-base and bedding depths follow the same shed-specific figures used above: 100mm of MOT Type 1 is the commonly quoted depth, on a 30mm bed of sharp sand the slabs are levelled into.
600 × 600mm is the default slab size here, the same UK-standard size the Paving Calculator defaults to, though 450 × 450mm and 600 × 300mm are common alternatives and a custom size is always available.
Plastic grid system
Interlocking plastic grid panels are a newer, lightweight base type: modular squares that click together over a membrane, optionally filled with pea gravel for a stable, permeable finish. ProBase and similar UK shed-base grid kits sell 500 × 500mm panels, the size used here, and ProBase's own installation guide states roughly 10kg of 10mm pea gravel per grid when filled, the figure used directly rather than the far higher weight-per-panel some gravel-driveway coverage guides assume. A weed-suppressing membrane sits under the grids, the same as under paving slabs or hardcore.
Filling is optional. ProBase's own guidance notes that for small or medium bases the grids are strong enough to leave unfilled, though most people fill them anyway for a tidier, more stable finish once the shed floor sits on top; this calculator lets that be switched off.
Timber frame on bearers
The lightest-duty of the four, and the one this calculator is most honest about: a treated timber perimeter frame with internal bearers spaced across it, sitting on a thin bed of compacted gravel or hardcore rather than a full sub-base. It suits a smaller, lighter shed well, since there's less to go wrong over a shorter span, but it's more prone to movement over time on soft ground than a slab, paved or grid base, and it's rarely the right choice for a large workshop-sized building.
Bearers are typically 100 × 47mm or 150 × 47mm pressure-treated timber, run across the base at either 400mm centres, the standard spacing, or 600mm centres, an economy spacing that suits only a light, small shed. This calculator counts a perimeter frame around the full base plus internal bearers at the chosen spacing, each cut to the base's width.
Worked example
Take the default 2.44 × 1.83m shed (8 × 6ft) with a +50mm border: the base works out to 2.54 × 1.93m, 4.90 m². As a concrete slab at 100mm on 100mm of hardcore, that's 0.49 m³ of concrete and 0.49 m³ of hardcore, about 1.03 tonnes of MOT Type 1, plus 5.39 m² of DPM membrane. In 600 × 600mm paving slabs with a 5mm joint, the same base needs 14 slabs before waste, 16 with 10% added for cuts, on 100mm of MOT and a 30mm sand bed. As a plastic grid system, that's 20 panels before waste, 22 to buy, needing about 200kg of pea gravel (8 bags) if filled. On a timber frame at 400mm centres, that's 8.94m of perimeter frame plus 5 internal bearers at 1.93m each, 18.6m of timber in total, on a 75mm gravel bed.
Questions people ask
Which base type suits which shed size?
A timber frame on bearers suits a small, light shed best, since it's the least resistant to movement over a longer span. Paving slabs and plastic grid systems both work well across most garden-shed sizes and are easier to lift or adjust than concrete. A concrete slab is the most permanent option and the one worth choosing for a larger workshop-sized building or anywhere that needs to carry real weight.
Do I need planning permission for a shed base?
Usually not. In the UK, a shed base on its own rarely needs planning permission, and most garden sheds fall under permitted development rules covering outbuildings, subject to size and position limits (such as not covering more than half the garden, or sitting forward of the house). Anything larger, closer to a boundary, or on a listed property or in a conservation area is worth checking with the local planning authority first.
Do I need a damp membrane under a shed base?
For a concrete slab, yes: a polythene DPM between the hardcore and the slab stops ground moisture wicking up into the concrete and, from there, into a timber shed floor. Paving slabs and plastic grid systems both benefit from a membrane too, mainly to suppress weeds and stop the sub-base being pressed down into soft ground, though it's a little less critical there than under a sealed concrete pour.
How level does a shed base need to be?
As close to dead level as the base type allows. Most shed manufacturers quote a tolerance of around 15–20mm across the whole base before doors and windows start to bind or roof panels stop meeting properly, and a concrete slab or full paved base is far easier to get within that than a grid or timber frame on uneven ground.
Should the base be the same size as the shed or bigger?
Most guides recommend 50–150mm bigger on every side, for drainage, a squaring-up margin and easier levelling. A smaller group argues a concrete base should stay flush with the shed instead, so rain running off the roof lands on the ground beyond the slab rather than pooling on an exposed shelf right next to the wall. This calculator offers both, since the right call depends on the ground and how exposed the site is.
Can a plastic grid base be left unfilled?
For a small or medium shed, often yes; the interlocking grids are strong enough on their own once bedded down. Most people fill them with pea gravel anyway for a tidier, more stable, better-draining finish once the shed floor sits on top, which is why this calculator defaults to filled but lets that be switched off.
Depths, densities and product figures are checked against at least two independent published shed-base guides for each base type, named above (ConcreteMath, Greenhouse Stores, SitePrep, MaterialCalculator and ProBase's own installation guide among them); the MOT Type 1 (2.1 t/m³) and sharp sand (1.6 t/m³) densities match this site's Paving and Hardcore calculators. Real depths and quantities vary by shed design, ground conditions and supplier, so treat these figures as a solid ordering estimate and confirm exact quantities, and any planning question, before a large order or build.