Garden Room Base & Slab Calculator

Works out the concrete slab and hardcore sub-base volumes for a garden room base from the footprint and layer depths. It runs entirely in your browser and shows the working, so you can check every step against your own figures.

How the garden room base & slab calculator works

The footprint is length times width. The concrete volume is that footprint times the slab depth; the hardcore sub-base volume is the footprint times the sub-base depth. Both are returned in cubic metres so you can order ready-mix and aggregate directly. A modest slab on a compacted sub-base suits most lightweight garden rooms.

What you enter

Enter the base length and width and the slab and sub-base depths; the calculator returns the concrete and hardcore volumes. Every field is editable, and the result recalculates the moment you change a value.

Reading the result

Order concrete about 10% over the calculated volume for spillage and an uneven sub-base. The hardcore figure is the compacted volume — loose aggregate bulks up, so buy a little extra.

Safe by design

This tool counts materials and cost. It never sizes the structural frame or the foundation loads — where those matter, a structural engineer signs them off.

Assumptions and limits

This sizes a plain slab and sub-base by volume. It does not design reinforcement, thickenings under load-bearing walls, or the foundation for the ground conditions — those are a groundworker or structural engineer’s call for anything beyond a light structure.

Using it with the rest of BuildMetricLab

This calculator is one step in planning the job. Pair it with the related tools in this category and across BuildMetricLab to go from a single figure to a full costed plan — a sizing figure feeds a cost estimate, a cost estimate feeds a project budget, and each result carries straight into the next tool. Every BuildMetricLab tool runs entirely in your browser — no sign-up, nothing sent to a server, and the formula is shown on the page so the maths can be audited.