Rebar Calculator
Bar counts including the lap splices most calculators forget.
Getting your rebar order right
Bars come in fixed lengths and every joint between them needs an overlap, so the quantity you measure and the quantity you buy are different numbers.
Why lap splices change the number
A 20 ft bar does not reach across a 28 ft slab, so two bars overlap in the middle. The overlap is not a token few inches — a common residential rule is forty times the bar diameter, which on #4 bar works out at 20 inches at every joint.
On a slab with twenty runs in each direction, that is a meaningful quantity of steel that a simple length-divided-by-spacing calculation never accounts for. It is the single most common reason a rebar order comes up short.
Lap length is not a fixed number. It changes with concrete strength, bar grade, whether the bar is coated and where in the pour it sits. The figure above is a starting point for ordering, not a specification — your local code and your engineer’s drawings decide the real one.
Rebar only works where it sits
Steel does its job in the middle third of the slab depth. Laid flat on the ground and pulled up with a hook as the concrete goes in, it rarely ends up there — it ends up near the bottom, where it contributes far less than the drawings assumed.
Chairs and bolsters hold the grid at the right height while the pour happens. They cost very little relative to the concrete and the steel, and they are the difference between reinforcement that works and reinforcement that is merely present.
Related calculators
- concrete calculator — work out the pour itself.
- gravel calculator — the sub-base under the slab.
- paver calculator — if you are laying rather than pouring.
FAQFrequently asked questions
Work out the bar runs in each direction from your grid spacing, then add a lap splice wherever two bars meet in a run. A 20 × 30 ft slab on a 16 inch grid needs bars both ways, and because the runs are longer than a standard 20 ft bar, several joints need overlapping. Enter your dimensions above and the calculator includes the laps.
Residential slabs commonly use #3 or #4 bar, with #4 the more frequent choice for driveways and anything carrying vehicle weight. The right answer depends on what the slab is for, the ground underneath and your local code, so treat any general rule as a starting point and confirm against your drawings.
| Bar size | Diameter |
|---|---|
| #3 | 3/8 inch |
| #4 | 1/2 inch |
| #5 | 5/8 inch |
| #6 | 3/4 inch |
Residential slabs are commonly placed on a 12 to 18 inch grid, with 16 inches a frequent default. Tighter spacing uses more steel and gives more crack control. The spacing you actually need comes from the slab's purpose and your local code rather than from a rule of thumb.
A common residential rule is forty times the bar diameter, so #4 bar overlaps by about twenty inches at each joint. The real figure varies with concrete strength, bar grade, coating and position in the pour. The calculator applies a default you can change, and shows the total lap allowance separately so you can see what it added.
It depends on what the slab carries and what your local code requires. Light-duty slabs are sometimes poured with welded wire mesh or fibre reinforcement instead. Anything carrying vehicle weight or spanning poor ground generally wants bar. This is a question for your local building department, not a calculator.
Weight per foot rises steeply with bar size, so a job in #5 weighs substantially more than the same job in #3. The calculator shows total weight alongside the bar count, which matters for transport and for whether the load fits in a pickup or needs delivery.
| Bar size | Weight per foot |
|---|---|
| #3 | 0.376 lb |
| #4 | 0.668 lb |
| #5 | 1.043 lb |
| #6 | 1.502 lb |
Roughly the middle third of the slab depth is where it does most good. Bar lying on the ground contributes very little. Chairs or bolsters hold the grid at height through the pour, which is the reliable way to get it there — lifting the mesh by hand as the concrete goes in generally is not.