Rebar Calculator — bars, laps and spacing check
Bar count and weight for a slab, including the lap splices most calculators omit and a check against the ACI 318 maximum spacing.
Bar count and weight, plus the two things most rebar calculators leave out: the lap splices that make your order longer than the slab, and whether your spacing passes the code maximum.
Quick answer: rebar weight by bar size
| Bar | Diameter | lb/ft | 20 ft bar | kg/m |
|---|---|---|---|---|
| #3 | 0.375 in. | 0.376 | 7.52 lb | 0.56 |
| #4 | 0.5 in. | 0.668 | 13.36 lb | 0.994 |
| #5 | 0.625 in. | 1.043 | 20.86 lb | 1.552 |
| #6 | 0.75 in. | 1.502 | 30.04 lb | 2.235 |
ASTM A615 nominal weights, the ones the calculator uses; the bar number is the diameter in eighths of an inch. Laps add length the slab never sees, so order on the calculator's lap-inclusive total, not on the slab size.
| Bars each way | — |
|---|---|
| Net bar length | — |
| Lap splice length | — |
| Total length to buy | — |
| Stock bars | — |
| Weight | — |
| Code maximum spacing | — |
The bar number is the diameter in eighths of an inch, up to #8. A #4 is 4/8 = 1/2 in. across and a #8 is a full inch; from #9 up the rule is only approximate (#9 is 1.128 in.). The nominal weights are ASTM A615 figures, and they rise with the square of the diameter: #6 weighs four times #3, not twice. The selector above stops at #6 because that is what a slab on grade takes; the table below carries the sizes a footing or a beam schedule names.
ASTM A615 bar sizes beyond the selector
| Bar | Diameter | lb/ft | kg/m | Area in² |
|---|---|---|---|---|
| #3 | 3/8" (9.53 mm) | 0.376 | 0.560 | 0.11 |
| #4 | 1/2" (12.7 mm) | 0.668 | 0.994 | 0.20 |
| #5 | 5/8" (15.9 mm) | 1.043 | 1.552 | 0.31 |
| #6 | 3/4" (19.1 mm) | 1.502 | 2.235 | 0.44 |
| #7 | 7/8" (22.2 mm) | 2.044 | 3.042 | 0.60 |
| #8 | 1" (25.4 mm) | 2.670 | 3.973 | 0.79 |
| #9 | 1.128" (28.7 mm) | 3.400 | 5.060 | 1.00 |
| #10 | 1.270" (32.3 mm) | 4.303 | 6.404 | 1.27 |
| #11 | 1.410" (35.8 mm) | 5.313 | 7.907 | 1.56 |
Weight is what you are billed for, and laps are inside it. Steel is sold by the ton, so every lap in the total is paid steel that spans nothing. On a 30 × 20 ft slab at 12 in. each way with 20 ft stock, the long bars lap once each; at 40 ft stock they do not lap at all, and the order drops by one lap length per bar. Checking stock length against the slab's long side before ordering is the cheapest saving on the page.
What I check on a job
Holding the count against the drawings. Before this count goes on an order, I check each input against the drawings' reinforcing notes. The page takes one spacing both ways; when the drawings give different spacings for the long and short bars, I run it once for each and take each direction's bar count from its own run. The stock and weight lines mix both directions, so for the order I work each direction's length out from its own bar count. The lap goes in as diameters, so a lap given in inches gets divided by the bar diameter first: 20 inches on a #4 is 40. I ask the supplier which stock length they deliver before choosing 20, 40 or 60 feet, because that sets the number of laps. Where the number runs light is the offcuts. The stock count is total length divided by bar length, rounded up, which assumes every leftover piece gets used. When runs don't divide evenly into the stock, some become scrap, so I treat the count as a floor.
Bar weights are the standard nominal figures — #3 at 0.376, #4 at 0.668, #5 at
1.043 and #6 at 1.502 lb per foot. Bars each way are (dimension − 2 × cover) ÷ spacing + 1.
Maximum spacing for slab reinforcement under ACI 318 is the lesser of three times the slab
thickness or 18″, so a 4″ slab caps at 12″ on centre. That is a maximum, not a design:
the actual size and spacing come from the engineer of record, and this page only tells you whether the
number you typed is inside the ceiling.
Lap length is entered here as a multiple of bar diameter because the real value depends on
concrete strength, bar grade, coating, position and confinement. Forty diameters is a common
placeholder, not a specification — take the lap from your drawings. This estimates material for
a flat slab on grade; footings, walls, beams, hooks, chairs and tie wire are separate.
Worked example
What is the lap splice for #4 bar at 40 diameters?
- A #4 bar is 4/8 = 0.500 in across.
- 40 × 0.5 = 20 in.
- Every bar that has to be spliced costs 20 in of extra length; the page adds it to the stock count.
20 inches (1'-8") per splice. For the 30 × 20 ft slab at 12 in both ways the page counts the bars, the laps and the stock lengths, and checks the spacing against the ACI 318 maximum.
2 values on this page were driven in a browser and compared with independently derived figures on 2026-10-29 (2808 values across 228 calculators site-wide).
Questions
How long is a 40-diameter lap splice for #4 bar?
20 inches. A #4 bar is ½ inch across, so 40 × 0.5 = 20.
What does the spacing check compare against?
The ACI 318 maximum spacing for slab reinforcement, so a spacing that computes fine but exceeds the limit is flagged.
How much does #8 rebar weigh?
2.670 lb per foot (3.973 kg/m); it is 1 in. in diameter, because up to #8 the bar number is the diameter in eighths of an inch. #9 is 1.128 in. and 3.400 lb/ft. Weight rises with the square of the diameter, so #6 weighs four times #3.
Does longer stock really reduce the order?
Yes, by the laps. On a 30 × 20 ft slab at 12 in. each way with 20 ft stock, every long bar laps once; with 40 ft stock none do, and the order drops by one lap length per bar. Check the stock length against the long side before ordering.