Rebar Calculator — Bar Count, Length & Weight
Who this is for: For concrete crews and DIY slab builders turning a spacing spec (like #4 at 18 inches each way) into an order list — how many sticks to buy and what the steel will weigh.
Slab rebar grid: enter dimensions, spacing, and bar size — get bar count per direction, total linear feet, 20-ft sticks, and weight.
Quick answer: Bars per direction = floor(width ÷ spacing) + 1 (after subtracting edge cover). A 10×12 slab with #4 at 18 inches OC needs 7 + 8 = 15 bars, about 166 linear feet ≈ 9 twenty-foot sticks ≈ 111 lb of steel.
18 in residential default
2-3 in typical
| Default spacing | 18 in on center (#4, 4-in slab) |
|---|---|
| Edge cover | 3 in typical |
| #4 weight | 0.668 lb/ft (20-ft stick = 13.4 lb) |
| Lap splice rule | 40 × bar diameter |
How does this rebar calculator work?
The grid is set by two numbers: your spacing on center and the edge cover (concrete between the outside bar and the form edge — 2 to 3 inches is typical). Bars in each direction = floor(available width ÷ spacing) + 1; the calculator lays both directions, totals the linear feet, converts to 20-foot sticks with lap splices in mind, and multiplies by unit weight for the tonnage (#4 weighs 0.668 lb/ft). Tie points estimate where two bars cross and get wired — roughly the intersections of the grid minus the perimeter edges. The weight matters because suppliers price by the hundredweight and your truck bed has limits.
Common uses
- Converting a '#4 @ 18 inches each way' spec into a materials order
- Estimating steel weight for pickup-truck logistics
- Checking lap-splice stick counts before the pour day
- Budgeting reinforcement alongside the concrete volume