Rebar Weight & Spacing Calculator
Take off the steel for a grid mat: enter the slab or footing area, the on-center spacing, and the bar size, and get bars each way, total bar length, and total weight at ASTM A615 nominal pounds per foot — then print it as a material order sheet. A takeoff tool, not a design tool: the bar size and spacing come from your structural drawings. Free, no login.
Grid / Mat
Rebar
Bars both directions at spacing; excludes laps, hooks & chairs. Empirical planning figures — confirm against the structural drawings.
Rebar Calculator — Material Order Sheet
Prepared with NORDIX — nordixhq.com/tools/rebar-calculator
| Grid: 20 × 20 ft | Spacing: 12" o.c. | Bar size: #4 |
| Material | Qty | Unit | Notes |
|---|---|---|---|
| #4 rebar | 42 | bars | 21 × 21 each way |
| Total bar length | 840 | ft | before laps |
| Total weight | 561.1 | lb | ASTM A615 nominal lb/ft |
Method: grid mat both ways at the entered spacing; ASTM A615 nominal bar weights. Excludes laps, hooks, chairs, and dowels — empirical planning figures; confirm counts and lap lengths against the structural drawings before ordering.
How this calculator works
A grid mat is counted the way it is laid out: bars running each direction at the entered spacing, plus one bar to close the run, with every bar spanning the full dimension:
bars each way = ⌊dimension ÷ spacing⌋ + 1 · weight = total length × lb/ft
Total length is both directions added together, and weight uses the ASTM A615 nominal bar weights — the same standard values every mill and supplier prices from (#4 = 0.668 lb/ft, #5 = 1.043 lb/ft). What the count deliberately excludes: lap splices, hooks, dowels, corner bars, and chairs. Bars come in stock lengths (20 ft is the most common), so any run longer than a stick laps per the drawings — typically 40 bar diameters or more — and laps plus hooks can add 10% or more to a long-run takeoff. Add them from the drawings before ordering.
Worked example
A 20 × 20 ft slab mat, #4 bars at 12" o.c. each way:
- Bars each way: ⌊20 ÷ 1⌋ + 1 = 21 × 21 — 42 bars total.
- Total length: 21 × 20 ft each direction = 840 ft.
- Weight: 840 × 0.668 lb/ft = 561.1 lb — about 42 twenty-foot sticks before laps.
Enter the same mat above (20 × 20 ft, 12" spacing, #4) to reproduce it.
Reference: ASTM A615 nominal bar sizes and weights
The bar table behind the calculator. The size number is the nominal diameter in eighths of an inch; weights are the ASTM A615 nominal values.
| Bar size | Nominal dia. | Weight (lb/ft) | 20-ft stick (lb) |
|---|---|---|---|
| #3 | 0.375" | 0.376 | 7.52 |
| #4 | 0.5" | 0.668 | 13.36 |
| #5 | 0.625" | 1.043 | 20.86 |
| #6 | 0.75" | 1.502 | 30.04 |
| #7 | 0.875" | 2.044 | 40.88 |
| #8 | 1" | 2.670 | 53.4 |
| #9 | 1.125" | 3.400 | 68 |
| #10 | 1.25" | 4.303 | 86.06 |
| #11 | 1.375" | 5.313 | 106.26 |
Frequently asked questions
How much does #4 rebar weigh per foot?
#4 rebar weighs 0.668 lb per foot — a standard 20-ft stick is about 13.4 lb, and a ton is roughly 150 sticks. Other common sizes: #3 is 0.376 lb/ft, #5 is 1.043 lb/ft, and #6 is 1.502 lb/ft. These are the ASTM A615 nominal weights, and they're the same values this calculator uses to convert total length to weight.
How do I calculate how many bars I need for a slab or mat?
For a grid mat, divide each dimension by the on-center spacing and add one for the closing bar — a 20 × 20 ft area at 12 in. o.c. takes 21 bars each way, 42 bars total. That's exactly how this calculator counts: bars running one direction span the full length, bars the other way span the full width, and total length is both directions added together. Total weight is that length times the bar's pound-per-foot value.
What size rebar and what spacing do I need?
That's an engineering call, not an estimating one. Bar size and spacing come from the structural drawings — they depend on loads, soil, concrete strength, and code, and only the engineer of record can set them. This calculator is a takeoff tool: enter the size and spacing the drawings specify and it counts the steel; it does not design the reinforcement.
Does this rebar calculator include lap splices?
No — the total length and weight are the theoretical grid only, excluding laps, hooks, dowels, and chairs. Rebar comes in stock lengths (20 ft is the most common), so any bar run longer than a stick needs lap splices, and laps are typically specified on the drawings as a multiple of bar diameter — often 40 diameters or more, around 20 in. for a #4. On long runs, laps plus hooks can add 10% or more to the takeoff, so add them per the drawings before you order.
What do rebar size numbers mean?
The number is the bar's nominal diameter in eighths of an inch: a #4 bar is 4/8 = 1/2 in., a #5 is 5/8 in., a #8 is a full inch. That's the US customary (ASTM A615) designation covering #3 through #11 in this calculator. Weight scales with the cross-section, which is why a #6 weighs more than twice a #4.
Method: bars each way = ⌊dimension ÷ spacing⌋ + 1, every bar spanning the full mat; weight from ASTM A615 nominal bar weights (#3–#11). The totals are the theoretical grid only — laps, hooks, dowels, corner bars, chairs, and cover adjustments are excluded and must be added from the structural drawings. Bar size and spacing are engineering decisions set by the engineer of record; this tool counts the steel you were told to place, it does not design reinforcement.
Pair with the Footing Calculator for footing concrete plus continuous-bar steel, the Construction Calculator for the slab’s concrete yardage, or the Block / CMU Calculator for the wall the dowels tie into.