Article — Rebar Calculator
Rebar Calculator: Bars, Linear Feet, and Weight for Concrete Slabs
A rebar calculator turns slab length, slab width, bar size, and spacing into the three numbers a concrete crew actually needs: bar count in each direction, total linear feet of steel (with a 10% lap allowance), and total weight in pounds. A 20 by 30 ft slab using #4 bar at 12 inches on center runs roughly 1,200 linear feet of steel and about 800 lb at ASTM A615 weights.
The math itself is plain arithmetic, but the inputs hide a lot of code. ACI 318-19 caps the spacing for shrinkage and temperature reinforcement at five times the slab thickness or 18 inches, whichever is smaller. ASTM A615 fixes the per-foot weight for every standard bar size in eighth-inch increments. The calculator stitches those rules together so you can size a pour from a tape measure and a pen.
What a rebar calculator does
A rebar calculator answers two questions at once. How many bars do I need in each direction, and how much steel is that in total? The first answer drives layout: where the bars sit on the chair supports, how wide the grid is, where you trim the last bar. The second answer drives buying: stick count at the supplier, freight weight on the trailer, and dollars on the invoice.
The form on this page asks for slab length, slab width, bar size, on-center spacing, and edge cover. It assumes a two-way grid, which is the standard pattern for slabs-on-grade. The bar count formula is floor(grid_dimension / spacing) + 1, which counts the bars including both edges. Total length multiplies bar count by slab dimension in the perpendicular direction, then adds 10% for lap splices between sticks.
The number in a rebar size is the bar’s nominal diameter measured in eighths of an inch. A #4 bar is 4/8 = 1/2 inch across. The convention only holds through #8 (1 inch); #9 through #11 are sized by cross-sectional area rather than physical diameter.
How the rebar calculator counts bars
The rebar calculator works in inches internally even when you type in feet. It subtracts twice the edge cover from each slab dimension to get the usable grid, then divides by the spacing and adds one. A 20 ft slab (240 in) with 3-inch edge cover and 12-inch spacing produces floor((240 - 6) / 12) + 1 = 20 bars across that side.
Bar direction matters for ordering. Bars that run along the length sit perpendicular to it — that is, they span across the width and you count them by walking along the width. The reverse holds for width-running bars. The form labels each output line with the slab dimension each bar covers, so confusing the two directions on the lumberyard order is hard to do.
Rebar sizes and weights
ASTM A615 fixes the per-foot weight for every standard US bar size. The numbers below are the ones the calculator multiplies by total linear feet to get the steel weight on the invoice.
- #3 = 0.376 lb/ft (3/8 in diameter)
- #4 = 0.668 lb/ft (1/2 in diameter)
- #5 = 1.043 lb/ft (5/8 in diameter)
- #6 = 1.502 lb/ft (3/4 in diameter)
- #7 = 2.044 lb/ft (7/8 in diameter)
- #8 = 2.670 lb/ft (1 in diameter)
The weight scales with the square of the diameter, which is why #5 (5/8 in) is over 50% heavier than #4 (1/2 in) per foot even though the diameter is only 25% larger. Cross-sectional area drives strength, and area grows with diameter squared.
4 in slab-on-grade #4 @ 12-18 in OC6 in driveway / patio #4 @ 12 in OCfooting under wall #5 @ 12 in OCretaining wall stem #5-#6 @ 12 in OCRebar spacing rules (ACI 318)
ACI 318-19 sets the upper bound on rebar spacing. For shrinkage and temperature reinforcement in slabs on ground, spacing is the smaller of five times the slab thickness or 18 inches. A 4-inch slab maxes out at 18 inches; a 5-inch slab could in theory go to 25 inches but stays capped at 18.
For structural reinforcement that resists flexure (the bars actually carrying load to columns or footings), the cap drops to three times the thickness or 18 inches. A 6-inch suspended slab tightens to 18 inches structurally and routinely runs 12 inches on a residential job to make placement and inspections cleaner.
“12 inches on center” means 12 inches from the centerline of one bar to the centerline of the next. The clear gap is 12 minus the bar diameter, so #4 bars at 12 in OC leave 11.5 in of clear space. Builders sometimes set bars at 12 in clear by mistake, which means they place fewer bars than the plan called for.
Estimating rebar by slab type
Three rough numbers handle 80% of residential estimates. A 4-inch slab with #4 at 12 in OC runs about 0.8 lb per square foot. The same slab with #4 at 18 in OC drops to about 0.55 lb per square foot. A 6-inch slab with #4 at 12 in OC in both directions is closer to 1.0 lb per square foot, even though the steel is the same. The extra weight comes from the cap on spacing relative to thickness.
For a real-world feel, a 24 by 24 ft attached garage slab at #4 / 16 in OC uses roughly 350-400 lb of rebar. A 1,200 sq ft basement floor with the same spec runs 800-1,000 lb. A 100-ft strip footing 16 in wide carries roughly 200 lb of #5 longitudinal bar plus ties.
Lap splice and development length
The rebar calculator adds a flat 10% to total linear feet for lap splices, which approximates Class B splices for slabs that exceed 20 ft. The full ACI formula for development length is L_d = (0.04 × d × f_y) / sqrt(f′c), where d is bar diameter in inches, f_y is the yield strength of the steel (60,000 psi for Grade 60), and f′c is the compressive strength of the concrete in psi.
For Grade 60 bar in 3,000 psi concrete that gives roughly 18 inches of development length for #4, 23 inches for #5, and 27 inches for #6. A Class B lap is 1.3 times that, so #4 at the same conditions splices over about 24 inches (2 ft). The 10% allowance baked into the calculator is generous for short slabs and conservative for long ones.
For exact splice math on long pours, run the formula once for your bar size and concrete grade, then take the percentage of total run length where two sticks overlap. On a 60 ft slab with 20 ft sticks you have two splice points per run, each consuming 24 inches — a 6.7% overhead, not 10%.
Common rebar mistakes
The mistakes that recur on every job: confusing on-center with clear spacing; forgetting that bars sold in 20-ft sticks need a splice to span a 25-ft slab; ignoring edge cover (ACI 318 requires 3 inches against earth, 1.5 inches elsewhere on slabs); and treating bar size like a strength rating rather than a diameter spec. Going from #4 to #5 doubles per-foot cost but only adds 55% to cross-sectional area, so unit cost per area-strength rises.
Cost of rebar per pound
US Bureau of Labor Statistics producer price data tracks Steel Mill Products (BLS series WPU101). Mill-net rebar pricing has averaged $0.30-$0.55 per pound at the fabricator gate, with regional retail premiums pushing $0.60-$0.80 per pound at a local supplier. A 1,000 lb residential order is roughly $400-$700 in material before delivery.