Bars, feet and pounds for the pours people actually build — and the one formula behind every row
← Back to the calculator“How much rebar do I need” is three questions in one: how many bars, how many feet, how many pounds. One formula answers all three, and the answer scales with the slab — so the table below is the same formula run for the pours most people are actually pouring. Find your size, read the row, then run the three lines for your own dimensions.
The basis for every number on this page: #4 bar at 12 in o.c. both ways, 1 in edge distance, 4 in slab. Change the spacing or the bar size and the number moves — the last two tables show how much.
Every row is the same reinforcement: #4 bar, 12 in on centre, both ways, 1 in edge. That is the common residential slab mat — patios, shed pads, garage floors, driveways and walkways. Read the row for your size, or use the two rows nearest it and interpolate the arithmetic yourself; the spacing page shows what each column is doing.
| Project | Size (ft) | Bars each way | Bars total | Length (ft) | Weight (lb) |
|---|---|---|---|---|---|
| Walkway | 4 × 20 | 4 + 20 | 24 | 160 | 106.9 |
| Shed slab | 10 × 12 | 10 + 12 | 22 | 240 | 160.3 |
| Patio slab | 12 × 12 | 12 + 12 | 24 | 288 | 192.4 |
| Small pad | 16 × 16 | 16 + 16 | 32 | 512 | 342.0 |
| Garage slab | 20 × 20 | 20 + 20 | 40 | 800 | 534.4 |
| Driveway | 12 × 24 | 12 + 24 | 36 | 576 | 384.8 |
| Wide driveway | 20 × 24 | 20 + 24 | 44 | 960 | 641.3 |
Swipe the table sideways for more columns →
People ask “how much rebar do I need” meaning three different things, and a good answer gives all three at once, because you need all three at different points:
For the 20 ft × 20 ft slab the three answers are 40 bars, 800 ft and 534.4 lb. They are not three calculations — they are one calculation read at three points. The weight page takes the pounds end further, and the tonnage page converts pounds into the tons a yard invoices in.
There is no lookup table you have to trust. Every row above is three lines of arithmetic, and you can run them for any rectangle:
The + 1 in line 1 is the bar that sits at the far edge. It is easy to leave out and it makes the count one bar short in every row, which on a 20 ft slab is 20 ft of steel. The clear span is the slab less the edge distance at both ends — measure to where the first bar can sit, not to the concrete face.
Worked on the 20 ft × 20 ft slab, one direction:
If the two directions are the same length the two counts are the same, which is why a square slab is easy to check by hand. On a rectangle the two counts differ, and the length has to pair each count with the other dimension — 12 bars running the 24 ft way plus 24 bars running the 12 ft way, not 12 × 12 plus 24 × 24.
A 20 ft × 20 ft slab on grade, 4 in thick, #4 bar at 12 in o.c. both ways, a 1 in edge to the first bar, 1.5 in of cover below. This is the garage-slab row of the table, run in full.
| Step | Number | Where it comes from |
|---|---|---|
| Clear span | 238 in | 240 in − 2 × 1 in edge |
| Bars, one direction | 20 | floor(238 ÷ 12) + 1 |
| Bars, both directions | 40 | 20 + 20 |
| Total length | 800 ft | 40 × 20 ft |
| Total weight | 534.4 lb | 800 ft × 0.668 lb/ft |
| Per square foot | 1.336 lb | 534.4 lb ÷ 400 sq ft |
Swipe the table sideways for more columns →
One thing worth noticing about this size: each bar is 20 ft long and the stock length is 20 ft, so the bars fit the stock exactly. There is no splice, no offcut, and the order is simply 40 lengths of 20 ft — 800 ft, 534.4 lb. Most sizes are not that tidy. A 12 ft bar does not pair two-to-a-length on 20 ft stock, and a 24 ft run does not fit 20 ft stock at all, which is where laps and offcut come in. That is the estimator page's job, and it is the difference between the number you count and the number you buy.
The row above is one spacing. Spacing is the input that moves “how much” the most, and it moves the length and the weight in step. Same 20 ft × 20 ft slab, same #4 bar, four spacings:
| Spacing | Bars each way | Bars total | Length (ft) | Weight (lb) | lb per sq ft |
|---|---|---|---|---|---|
| 12 in o.c. | 20 | 40 | 800 | 534.4 | 1.336 |
| 16 in o.c. | 15 | 30 | 600 | 400.8 | 1.002 |
| 18 in o.c. | 14 | 28 | 560 | 374.1 | 0.935 |
| 24 in o.c. | 10 | 20 | 400 | 267.2 | 0.668 |
Swipe the table sideways for more columns →
Doubling the spacing from 12 in to 24 in halves the bars, the feet and the pounds — exactly, because the + 1 is the same bar in both. The quick cross-check is lb per sq ft = 2 ÷ spacing in feet × lb per foot, which at 12 in gives 2 ÷ 1 × 0.668 = 1.336 and at 24 in gives 0.668. Treat that as a check, not the answer: it ignores the end bar and the edge trim, so it drifts a few percent either way from the counted total, and it says nothing about which spacing your slab actually needs.
The takeoff is what is in the slab. The order is what leaves the yard, and the two differ by the cutting remainder, because steel comes in whole stock lengths. Three things follow, and none of them change the takeoff:
A 20 ft × 20 ft slab in #4 bar at 12 in on centre both ways, with a 1 in edge, takes 20 bars each way. That is 40 bars, 800 ft of bar and 534.4 lb, or 1.34 lb per square foot. The count is floor(clear ÷ spacing) + 1 per direction: the clear span is 20 ft minus a 1 in edge each side = 238 in, and floor(238 ÷ 12) + 1 = 20 bars. Because each bar is 20 ft long, 20 ft stock lengths fit with no splice and no offcut: buy 40 lengths.
For #4 bar at 12 in on centre both ways it is 1.336 lb per square foot, and that number holds for every slab size: a 10 × 10 ft slab is 133.6 lb, a 12 × 12 ft patio is 192.4 lb and a 20 × 20 ft slab is 534.4 lb, all 1.336 lb per square foot. The shortcut is lb per sq ft = 2 ÷ spacing in feet × lb per foot, so at 12 in it is 2 ÷ 1 × 0.668. The shortcut ignores the extra end bar and the edge trim, so treat it as a cross-check and use the counted total when the number matters. The weight page carries the per-square-foot table for #3 to #8 across five spacings.
Count one direction at a time: bars = floor(clear span in inches ÷ spacing in inches) + 1, where clear span = slab dimension minus the edge distance at both ends. Do it for both directions and add them. On a 20 ft × 20 ft slab in #4 bar at 12 in o.c. with a 1 in edge that is floor(238 ÷ 12) + 1 = 20 bars each way, so 40 bars in total. The + 1 is the bar that sits at the far edge; forgetting it is the most common way a count comes out one bar short per row.
There is no fancy way, and that is the honest answer. Total rebar is three lines: bars each way = floor(clear ÷ spacing) + 1, total length = sum of (bar count × run length), and total weight = length × lb per foot. Everything else — laps, offcut, stock length, waste — is a correction applied afterwards. A 20 × 20 ft slab is 20 + 20 bars, 40 × 20 ft = 800 ft, and 800 × 0.668 = 534.4 lb. If a tool gives you a different total for the same inputs, ask it which of the three lines it changed.
A 12 ft × 12 ft patio slab in #4 bar at 12 in on centre both ways with a 1 in edge takes 12 bars each way: 24 bars, 288 ft and 192.4 lb. Clear span is 144 in minus 2 in = 142 in, and floor(142 ÷ 12) + 1 = 12 bars. Note that the 12 ft bar does not nest two to a 20 ft stock length (two would be 24 ft), so the order is one 12 ft bar per 20 ft length unless you step up to 40 ft stock, where three bars fit with 4 ft left over. Ordering is covered on the rebar estimator page.
A 10 ft × 10 ft slab in #4 bar at 12 in on centre both ways with a 1 in edge takes 10 bars each way: 20 bars, 200 ft and 133.6 lb. Clear span is 120 in minus 2 in = 118 in, and floor(118 ÷ 12) + 1 = 10 bars. At 1.336 lb per square foot a 100 sq ft slab is 133.6 lb, which is the same figure the per-square-foot shortcut gives.
You order by the bar and you are invoiced by the pound. Suppliers ship whole stock lengths — commonly 20 ft or 40 ft — so the order is a bar count, but the invoice and the price per ton are weight, which is the bar count times the length times lb per foot. Give the yard both numbers: 40 bars of 20 ft #4 is also 800 ft and 534.4 lb. The weight calculator runs that conversion per foot, per bar and per order.
Add nothing to the takeoff, and do not use a percentage. The takeoff above is the steel in the finished grid. What you actually buy is whole stock lengths, so the difference between the takeoff and the order is a cutting remainder, not a waste factor: it depends on how the run lengths divide into the stock length, and it ranges from 0% to 40% on 20 ft stock. A 20 × 20 ft slab on 20 ft stock happens to divide exactly, so the order and the takeoff are both 800 ft. The rebar estimator page works the remainder through line by line.