Construction Calculator
Rebar Calculator
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Work out the reinforcement a pour takes — bar counts in each direction, total linear feet and the weight it comes to, with concrete cover, lap splices, layers and a waste allowance.
Rebar Calculator
Bar count, linear length and weight
Bars run in both directions on a grid. Both sets are counted, each spaced across the perpendicular dimension.
Lap splices (optional)
No splices are counted. Pick a stock length to add lap allowance for bars longer than the stock.
Results
Total weight
899 lb
408 kg · #4
Linear length
1,346 ft
52 bars incl. waste
Bars by Direction
| Direction | Spaced across | Bars | Each | Total |
|---|---|---|---|---|
| Bars running the length | 29.50 | 31 | 19.50 | 604.5 |
| Bars running the width | 19.50 | 21 | 29.50 | 619.5 |
Bars running one way are spaced across the perpendicular dimension, which is what sets their count. Lengths shown are after deducting cover at both ends.
Weight at This Length by Bar Size
| Bar | Dia | lb/ft | Weight | Area/ft |
|---|---|---|---|---|
| #3 | 0.375 | 0.376 | 506 lb | 0.110 |
| #4 ← | 0.500 | 0.668 | 899 lb | 0.200 |
| #5 | 0.625 | 1.043 | 1,404 lb | 0.310 |
| #6 | 0.750 | 1.502 | 2,022 lb | 0.440 |
| #7 | 0.875 | 2.044 | 2,752 lb | 0.600 |
| #8 | 1.000 | 2.670 | 3,595 lb | 0.790 |
| #9 | 1.128 | 3.400 | 4,578 lb | 1.000 |
| #10 | 1.270 | 4.303 | 5,794 lb | 1.270 |
| #11 | 1.410 | 5.313 | 7,153 lb | 1.560 |
Weight assumes the same total length, which is only true if the spacing stays put. Area per foot is the figure a drawing specifies, so it is what to check a substitution against.
Bar counts use (span ÷ spacing) + 1, rounded up, because a partial gap at the end of a run still needs its bar. The "+1" is the bar at the far edge: a 20 ft span at 12 in centres has 20 gaps and 21 bars.
Bars running one way are spaced across the other dimension. Here that gives 31 and 21 bars — the 31 are spaced across 30.00 ft and each runs 19.50 ft.
#4 is 0.5 in diameter, 0.2 sq in in area and 0.668 lb/ft, from the ASTM A615 nominal table. These are published values rather than derived ones: computing weight from the nominal area and steel density is off by up to 0.019 lb/ft, and the familiar "number over eight" rule for diameter stops holding above #8 — #9 is 1.128 in, not 1.125.
Steel area works out to 0.200 sq in per foot of width at 12 in centres. That figure is what a structural drawing specifies, so it is the number to check a substitution against — a different bar size at a different spacing can supply the same area.
Weights are nominal. Mill-certified weight varies slightly, and rebar is normally sold by weight, so the invoice figure may differ a little from this estimate.
This estimates quantity and weight only. Bar size, spacing, cover, lap length and the number of layers all come from a structural drawing — they follow from the loads, the span, the exposure and the code, and none is a quantity question.
Step-by-Step Calculation
Step 1 — Bar counts
cover of 3 in is deducted at each edge
Bars running the length:
span 30.000 − 2 × 0.25 = 29.500 ft
(29.500 ÷ 1) + 1 = 30.5000 → 31 bars
each 19.500 ft long → 604.50 ft
Bars running the width:
span 20.000 − 2 × 0.25 = 19.500 ft
(19.500 ÷ 1) + 1 = 20.5000 → 21 bars
each 29.500 ft long → 619.50 ft
Step 2 — Total linear length
604.50 + 619.50 = 1,224.00 ft
Step 3 — Lap splices
No stock length given, so no splices are counted
Step 4 — Add 10% waste
1,224.00 × 1.1 = 1,346.40 ft
Step 5 — Weight
#4 weighs 0.668 lb/ft (ASTM A615 nominal)
1,346.40 ft × 0.668 = 899.40 lb
= 407.96 kg
Result — 52 bars, 1,346.4 ft, 899.4 lb (408.0 kg)
Counting Bars in a Rebar Grid
Bars are counted like fence posts: a span divided by the spacing gives the gaps, and you need one more bar than gaps. Bars that run the length of the slab are spaced across its width, and vice versa.
Count Bars, Not Gaps
Bar counts come from one formula: span ÷ spacing, plus one. The plus one is there because you are counting bars rather than the gaps between them — a 20 ft span at 12 inch centres has 20 gaps and 21 bars, the same way a fence 20 panels long needs 21 posts.
The part worth getting straight is which span goes with which direction. Bars running the length are spaced across the width, so on a 20 × 30 ft slab the 31 bars are the ones spaced across the 30 ft width, and each of them is 20 ft long. Swap those and the total length comes out wrong even though both counts look right.
From there it is arithmetic: 31 × 20 + 21 × 30 = 1,250 linear feet, and at 0.668 lb/ft for #4 bar that is 835 pounds.
Rebar Formulas
1. Number of Bars
2. Total Linear Length
3. Lap Splices
4. Weight and Waste
Rebar Sizes and Weights
ASTM A615 nominal values, with the weight of a 20 ft stock bar:
| Size | Diameter | Area | lb/ft | kg/m | 20 ft bar |
|---|---|---|---|---|---|
| #3 | 0.375 in | 0.11 in² | 0.376 | 0.560 | 7.5 lb |
| #4 | 0.500 in | 0.20 in² | 0.668 | 0.994 | 13.4 lb |
| #5 | 0.625 in | 0.31 in² | 1.043 | 1.552 | 20.9 lb |
| #6 | 0.750 in | 0.44 in² | 1.502 | 2.235 | 30.0 lb |
| #7 | 0.875 in | 0.60 in² | 2.044 | 3.042 | 40.9 lb |
| #8 | 1.000 in | 0.79 in² | 2.670 | 3.973 | 53.4 lb |
| #9 | 1.128 in | 1.00 in² | 3.400 | 5.060 | 68.0 lb |
| #10 | 1.270 in | 1.27 in² | 4.303 | 6.404 | 86.1 lb |
| #11 | 1.410 in | 1.56 in² | 5.313 | 7.907 | 106.3 lb |
The size number is the diameter in eighths of an inch — but only up to #8. The three bolded diameters show where that rule breaks: #9 is 1.128 in rather than 1.125, #10 is 1.270 rather than 1.250, and #11 is 1.410 rather than 1.375. Above #8 the sizes are set by area instead, which comes out at exactly 1.00, 1.27 and 1.56 square inches. These lb/ft figures are also published values rather than derived ones — working weight out from the nominal area and steel density is off by up to 0.019 lb/ft, worst at #10.
A Worked Example, and Where the Usual Figures Go Wrong
For a 20 × 30 ft slab with #4 bar at 12 inch centres both ways:
| Quantity | Correct | Often quoted | Why |
|---|---|---|---|
| Bars each way | 31 and 21 | 31 and 21 | Correct — (span ÷ spacing) + 1 |
| Total length | 1,250 ft | 1,260 ft | 620 + 630 = 1,250, not 1,260 |
| Weight | 835 lb | 668 lb | 668 is 0.668 × 1,000 — the rate times a round thousand |
The weight error is the one that costs money: 668 lb understates the real 835 lb by 25%, which is 167 pounds of steel. It comes from multiplying the lb/ft rate by a round 1,000 instead of by the actual length — and 1,000 is not a figure that appears anywhere in this slab. Even using the mistaken 1,260 ft length, the weight would be 841.68 lb. If you see a rebar weight that looks like the per-foot rate with the decimal point moved, check it.
Why Cover Shortens Bars but Rarely Removes Them
Bars start one cover distance from each edge, not at the edge. On the 20 × 30 ft slab with #4 at 12 in:
| Cover | Bars | Total length | Actual spacing |
|---|---|---|---|
| 0 in | 31 / 21 | 1,250.0 ft | 12.000 in |
| 1.5 in | 31 / 21 | 1,237.0 ft | 11.900 in |
| 2 in | 31 / 21 | 1,232.7 ft | 11.867 in |
| 3 in | 31 / 21 | 1,224.0 ft | 11.800 in |
| 4 in | 31 / 21 | 1,215.3 ft | 11.733 in |
| 6 in | 30 / 20 | 1,150.0 ft | 12.000 in |
The count holds at 31 all the way to 4 inches of cover, and only drops at 6. The reason is that spacing is a maximum, not a target: bars may sit closer than specified but never farther apart. Thirty bars across a 29.5 ft span would be 12.207 in apart, breaching a 12 in specification, so the count rounds up to 31 and the spacing tightens to 11.8 in instead. What cover reliably changes is length — 3 inches takes this slab from 1,250 ft to 1,224 ft, a 2.1% saving that is worth having on a large pour.
Steel Area Per Foot: The Number on the Drawing
A structural drawing specifies area of steel per foot of width, not a bar count. Square inches per foot, single layer:
| Bar | 4 in | 6 in | 8 in | 12 in | 18 in |
|---|---|---|---|---|---|
| #3 | 0.330 | 0.220 | 0.165 | 0.110 | 0.073 |
| #4 | 0.600 | 0.400 | 0.300 | 0.200 | 0.133 |
| #5 | 0.930 | 0.620 | 0.465 | 0.310 | 0.207 |
| #6 | 1.320 | 0.880 | 0.660 | 0.440 | 0.293 |
This is the table to use when a bar size is unavailable and a substitution is proposed. #4 at 6 in gives 0.400 and #5 at 9 in gives 0.413, so the #5 substitution covers the area with a little to spare. Halving the spacing exactly doubles the area, and a second layer doubles it again. Area is necessary but not sufficient, though — bar size and spacing also drive crack control and development length, so a substitution still needs sign-off from whoever engineered the pour.
Lap Splices and Stock Length
Any bar longer than the stock available has to be spliced. On the example slab, only the 30 ft bars are affected:
| Stock length | Splices | Total length | Bars to order |
|---|---|---|---|
| 20 ft | 21 | 1,285.0 ft | 65 |
| 30 ft | 0 | 1,250.0 ft | 42 |
| 40 ft | 0 | 1,250.0 ft | 32 |
| 60 ft | 0 | 1,250.0 ft | 21 |
Going from 20 ft to 30 ft stock removes all 21 splices and 35 feet of lap. Lap length itself is quoted as a multiple of bar diameter, and on this slab 40db adds 2.8%, 48db adds 3.4% and 60db adds 4.2%. Which multiple applies is not a choice — it comes from the drawing, and depends on the concrete strength, the bar grade, and whether the splice is in tension or compression. Treat the figures here as an allowance for ordering, not as a specification.
Example Calculations
A slab grid, a footing, and the same slab with splices and waste:
Example 1 — 20 ft × 30 ft Slab, #4 at 12 in Both Ways
The brief case, worked through. Two of its three answers do not hold up.
Bars across the 30 ft width: (30 ÷ 1) + 1 = 31, each 20 ft long
Bars across the 20 ft length: (20 ÷ 1) + 1 = 21, each 30 ft long
Bar counts of 31 and 21 match the brief exactly
Total length = 31×20 + 21×30 = 620 + 630 = 1,250 ft
The brief says 1,260 ft — 10 ft out
Weight = 1,250 × 0.668 = 835 lb, not the 668 lb stated
Example 2 — 40 ft Footing, #5 at 8 in
Underspecified in the brief: a footing needs a width to size its bars.
Transverse bars across 40 ft: (40 ÷ 0.667) + 1 = 61 bars
Each runs the footing width — which the brief never gives
Assuming a 2 ft wide footing: 61 × 2 = 122 ft of transverse steel
Plus longitudinal bars along the 40 ft run: 4 × 40 = 160 ft
Total 282 ft of #5 at 1.043 lb/ft = 294 lb
Change the width and both figures move — it is a required input
Example 3 — The Same Slab With Laps and Waste
What splices and a waste factor add to Example 1.
Base length 1,250 ft (no cover deducted)
From 20 ft stock, the 30 ft bars each need 1 splice: 21 splices
Lap at 40 bar diameters = 40 × 0.5 in = 1.667 ft each
21 × 1.667 = 35 ft of lap, giving 1,285 ft
With 10% waste = 1,413.5 ft and 944 lb
Laps added 2.8%; a 30 ft stock length would need none
Before You Order
Take the bar size, spacing, cover and lap length from the structural drawing rather than from any default here, including the ones on this page — those five values are the engineering, and a quantity estimate built on guessed values is only an estimate of the wrong thing. Check whether the spacing on the drawing is a maximum or a centre-to-centre dimension, since that decides whether to round the count up or place bars to suit. Watch out for anything the rectangle model misses: openings, column heads, edge thickenings and corner bars all change the count, and a slab with a stair opening needs the trim bars around it added by hand. Confirm what stock lengths your supplier actually carries before assuming a 20 ft bar, because moving to 30 or 40 ft stock can remove every splice on a job and the lap steel with it. Order by weight, since that is how rebar is sold, and expect the mill-certified figure to differ slightly from these nominal weights. Finally, this counts steel and weighs it — it is not a reinforcement design, and it does not check development length, crack control or anything else the code requires.
Frequently Asked Questions
- How do you calculate how much rebar you need?
- Divide the span by the spacing and add one, for each direction. A 20 ft × 30 ft slab at 12 inch centres needs (30 ÷ 1) + 1 = 31 bars running the 20 ft direction and (20 ÷ 1) + 1 = 21 running the 30 ft direction. Multiply each count by its bar length and add: 31 × 20 + 21 × 30 = 1,250 linear feet. Then multiply by the bar weight per foot for the total weight.
- Why do you add one to the span divided by spacing?
- Because you are counting bars, not gaps. A 20 ft span at 12 inch centres has 20 gaps between bars but 21 bars, the same way a fence 20 panels long needs 21 posts. Leaving the +1 off undercounts every direction by one bar.
- How much does rebar weigh per foot?
- From the ASTM A615 nominal table: #3 is 0.376 lb/ft, #4 is 0.668, #5 is 1.043, #6 is 1.502, #7 is 2.044, #8 is 2.670, #9 is 3.400, #10 is 4.303 and #11 is 5.313. These are published values, not derived ones — computing weight from the nominal area and steel density is off by up to 0.019 lb/ft.
- What do the rebar size numbers mean?
- The number is the diameter in eighths of an inch — but only up to #8. A #4 bar is 4/8 = 0.5 inches and a #8 is 1.0 inch, yet #9 is 1.128 inches rather than 1.125, #10 is 1.270 rather than 1.250, and #11 is 1.410 rather than 1.375. Above #8 the sizes follow the cross-sectional area instead: #9, #10 and #11 have nominal areas of exactly 1.00, 1.27 and 1.56 square inches.
- How many bars for a 20 × 30 slab at 12 inch spacing?
- 52 bars in a single layer — 31 one way and 21 the other. That comes to 1,250 linear feet and, in #4 bar, 835 pounds before any waste allowance. Two layers doubles both figures, to 104 bars and 1,670 pounds.
- Does concrete cover change the number of bars?
- Usually not, though it does shorten every bar. On a 30 ft width with 3 inch cover the span between outer bars drops to 29.5 ft, but the count stays at 31 — because spacing is a maximum, not a target. Thirty bars across 29.5 ft would sit 12.207 inches apart and breach a 12 inch specification, while 31 bars tighten to 11.8 inches and comply. What cover does change is length: each bar loses 6 inches, taking the example from 1,250 ft to 1,224 ft.
- How do you account for lap splices?
- Any bar longer than the stock length has to be spliced, so splices per bar is the bar length divided by the stock length, rounded up, minus one. Each splice adds a lap, commonly specified as 40 or 48 bar diameters — for #4 bar that is 1.667 or 2.0 feet. On the example slab from 20 ft stock, the 30 ft bars each need one splice, adding 35 feet or 2.8%. Required lap length comes from the drawing, since it depends on concrete strength, bar grade and whether the splice is in tension or compression.
- What waste factor should I use for rebar?
- 5 to 15% is the usual range, and it sits at the lower end of that for most jobs because offcuts can often be used elsewhere — a piece cut off a long bar becomes a short bar somewhere else. On the 1,250 foot example, 10% adds 125 feet and about 84 pounds.
- How do I check a different bar size gives the same strength?
- Compare steel area per foot of width, which is the figure a structural drawing actually specifies. #4 at 6 inch centres gives 0.400 square inches per foot; #5 at 9 inches gives 0.413, so it is a valid substitution on area. #4 at 12 inches gives only 0.200, half as much. Always confirm a substitution with whoever engineered the slab, since bar size and spacing also affect crack control and development length.
- Does this calculator design the reinforcement?
- No — it estimates quantity and weight. Bar size, spacing, cover, lap length and the number of layers all come from a structural drawing: they follow from the loads, the span, the exposure conditions and the code, and none of them is a quantity question. Weights are also nominal, and since rebar is sold by weight the invoice may differ slightly from mill-certified actuals.