Loan Rebar Estimator
A slab grid looks simple until you are standing in the yard: how many bars each way, how many 20-ft sticks that really is, and what half a ton of steel weighs on the trailer. Enter the slab and the spacing — get all three numbers.
Clearance keeps steel off the forms — 3 in is the standard cover where concrete casts against earth. The allowance covers lap splices (bars overlap about 24–30 in where they join) and cuts.
How it works
Bars are counted across the usable slab — the full dimension minus clearance at both edges. Bars running the length are spaced across the width: count = usable width ÷ spacing, rounded up, plus one for the far edge; same across the other direction. Total steel is bars × their lengths. Weight per foot is fixed by bar size: #3 = 0.376, #4 = 0.668, #5 = 1.043, #6 = 1.502 lb/ft. Sticks assume 20-ft stock with the lap-and-cut allowance folded in.
Formula & worked example
Formula: Bars each way = ceil((dimension × 12 − 2 × edge clearance) ÷ spacing) + 1. Total steel = bars × lengths; weight uses the standard lb/ft by bar size (#3 0.376, #4 0.668, #5 1.043, #6 1.502). Sticks are 20-ft with a lap and cutting allowance.
Worked example: A 20 × 20 ft slab at 12-in spacing in #4: 21 bars each way, 840 lineal feet, 47 sticks to buy, about 561 lb of steel.
When to use this
- You are reinforcing a slab, driveway or patio and ordering from a steel supplier.
- You need the trailer weight before hauling 20-ft sticks home.
- You are checking a contractor's steel list against the slab dimensions.
How to use it, step by step
- Enter slab length and width.
- Pick grid spacing for the duty — 12 in driveways, 18 in patios.
- Pick the bar size (#4 is the residential default).
- Check sticks AND total weight before ordering.
What the inputs mean
| Input | What it is | Typical values |
|---|---|---|
| Slab size | Length × width of the pour | 10 × 12 patio to 20 × 20 driveway |
| Grid spacing | Bar spacing each way | 12, 18 or 24 in |
| Bar size | Diameter in eighths of an inch | #3 light; #4 standard; #5 heavy |
| Edge clearance | Concrete cover between steel and form | 3 in against earth is standard |
Another worked example
A 12 × 10 ft patio slab at 18-in spacing in #3 bar: usable dimensions shrink by 3 in of clearance each side, giving 8 bars running the length and 9 across — 186 lineal feet, 11 sticks with laps, and only about 70 lb of steel. Light slabs are a pickup-truck job, not a delivery.
Frequently asked questions
How much rebar do I need for a 20 × 20 slab?
At 12-in spacing with #4 bar: 21 bars each way (the edges eat 3 in on both sides), 840 lineal feet of steel, 47 twenty-foot sticks with laps and cuts, weighing about 561 lb. At 18-in spacing the same slab drops to roughly 30 sticks.
What spacing is typical?
Twelve inches each way for driveways and slabs carrying vehicles; 18 in for patios and most residential slabs; 24 in only for light-duty work like sidewalks. When the soil is expansive or the load is real, tighter wins.
#4 or #5 bar?
#4 (½ in) is the residential default — easy to cut and bend, strong enough for slabs on grade. #5 shows up in footings, walls and heavier slabs. The number is the diameter in eighths of an inch: #5 = ⅝ in.
Rebar or wire mesh?
Mesh controls surface cracking in thin, light slabs and is faster to place; bar adds real bending strength and survives being walked on during the pour. Many pros use bar in driveways and mesh in patios — the Concrete Calculator sizes the pour either way.
How much lap do bars need?
A common planning figure is 40 bar diameters — about 20 in for #4 — but the allowance here (10%) is a purchase cushion, not a splice design. Follow your plans for actual laps.
🏗️ Sizing the pour itself?
Steel counted, the Concrete Calculator works out cubic yards, bag counts and cost for the slab — shapes from footings to steps included.
Do I need rebar in a 4-inch patio slab?
Strictly, fibre mesh or wire mesh can suffice for light patios on good soil; bar earns its place where soil moves, loads concentrate, or the slab is thick. This estimator sizes the bar route honestly either way.
How do I transport 20-ft sticks?
Suppliers cut and bundle for a fee, and most yards deliver. If you haul: a ladder rack or trailer with the bundle flagged — 47 sticks of #4 weigh 561 lb before the trailer notices anything else.