Rebar Calculator
Plan the reinforcement grid for a slab.
Enter the slab dimensions, bar spacing, edge cover and lap length, and get the number of bars in each direction, total linear feet, how many stock lengths to buy, and how many intersections need tying.
The calculation includes lap splices, which is where most hand estimates fall short.
How to use the rebar calculator
Enter the slab dimensions and bar spacing.
Set the edge cover and lap length.
Read bar counts, linear feet and stock bars needed.
Why use the Build Tally rebar calculator
Laps included
Splice overlaps are added to the total, not ignored.
Both directions
Bar counts lengthwise and widthwise separately.
Stock lengths
How many bars to actually buy.
Tie count
Number of grid intersections.
How a rebar grid is laid out
Bars run in two directions at a set spacing, usually 12 or 18 inches on centre, held back from the slab edge by a cover distance of 2–3 inches so the steel stays protected inside the concrete. The number of bars in one direction is the clear span divided by the spacing, plus one — the plus one catches the bar at the far edge, and forgetting it is the most common counting error.
Lap splices
Where two bars meet end to end they must overlap so the load transfers between them. A common rule of thumb for residential work is 40 bar diameters — about 20 inches for #4 bar and 25 inches for #5 — but the required lap depends on concrete strength, bar size and position, and your engineer or code will specify it. Laps add real quantity: on a large slab they can be 5–8% of total steel.
Chairs and tying
Rebar must sit in the middle third of the slab depth to do its job, which means supporting it on chairs or bolsters rather than pulling it up during the pour — a technique that rarely works. Every intersection does not need tying; typically every second or third one is enough to hold the grid stable. The calculator shows the total intersection count so you can judge how many ties to buy.
Read the guide: How Much Concrete Do I Need? A Straight Answer for Slabs, Footings and Posts
Frequently asked questions
How much rebar do I need for a slab?
It depends on spacing. Enter your slab size and bar spacing above and the calculator gives bar counts, linear feet and stock lengths including laps.
What spacing should rebar be?
Twelve to eighteen inches on centre is typical for residential slabs, but the correct spacing comes from your code or engineer.
How much should rebar overlap?
A common residential rule is 40 bar diameters — about 20 in for #4 and 25 in for #5 — but confirm against your specification.
How far from the edge should rebar be?
Usually 2–3 inches of cover, so the steel stays fully encased and protected from corrosion.
Do I need to tie every intersection?
No. Every second or third is usually enough to keep the grid stable during the pour.
Can I use wire mesh instead?
Welded wire mesh is common for light residential slabs. Rebar is used where more strength or crack control is needed.
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