Plan a slab
Every trade, one set of measurements
Slab planner
A slab is three orders that all come off one set of figures, and they do not scale together: the concrete by volume, the reinforcement by area, and the membrane, tape and perimeter insulation by what they wrap. Give it a length, a width and a thickness and each comes back from the calculator that owns it, with the volume — not the area — being what concrete is actually ordered by.
- 1 Measure the slab
- 2 Choose its finishes
- 3 Save it to My Project
Give the slab a length, a width and a thickness, and it is drawn here with every figure beside it.
The slab
Nothing here is required. Each figure appears as soon as it has what it needs.
Length, width and thickness. The thickness is asked for here, unlike on a deck or a drive, because the volume cannot be worked out without it and the volume is what you order.
Add a slab
A floor slab, a base or a pad. It is not under anything here — give it its own three figures and it is planned beside the rest.
Everything the slab needs
In the order the work is done. No row has a figure yet.
Nothing in this slab is everything the slab needs yet. Choose how a surface is finished, or what else is being done, and its rows appear here.
Assumptions assumed
Everything the figures above rest on that you did not give, and what they are still waiting for.
Nothing is assumed yet: give the slab its measurements to begin.
How the planner runs
- Give the slab its length, its width and how thick it is.
- Read the concrete as a volume, which is how it is bought, rather than as an area.
- Take the reinforcement over the slab's area at the spacing you are using.
- Add the membrane under it, the tape at its joints and the foam at its edge.
- Save it into your project, where the pour can be priced with everything that goes under it.
Where the figures come from
- Vapor Barrier Calculator
- Under-Slab Vapor Barrier Seam Tape Calculator
- Under-Slab Perimeter Rigid Foam Board Calculator
- Rebar Calculator
- Concrete Calculator
Reading for the decisions
Questions
- Why is the sub-base not in the list?
- How deep the ground has to be made up under a slab is a judgement about that ground — what it is, how it drains, what the slab will carry — and no rectangle decides it. Nothing is loaded for it and nothing links to it, which is the page's way of saying it is a real gap rather than an oversight.
- Is the thickness a safe default if I do not know it?
- No, and it is the input on this page that carries the most risk. A slab carrying a wall, a vehicle or a machine base is sized by an engineer against the ground it sits on; a thickness taken from a similar job is a guess about somebody else's soil.
- Does the concrete figure include waste?
- It carries the waste allowance the concrete calculator applies, which you can see and change on its own page. A slab poured against dug ground rather than formwork routinely takes more than the geometric volume, and that is worth adding deliberately.
- Why is the membrane area larger than the slab?
- It turns up at the edges and laps at the joints, and both are material. The seam tape is counted along those joints, which is why the two figures move together.
When the work is a job rather than a room. Plenty of it never starts from a space — a slab, a roof, a deck, a panel upgrade. Each of those is a job takeoff: give it the few figures that job is described by, and every trade in it comes back together, the way the room does here.