Rebar calculator
Enter a slab, footing, or pier and this lays out the reinforcing steel: how many bars, how long each one is after cover, the total feet of rebar, and how many stock lengths to buy. It runs the same layout math as the CutOnce app, which figures it beside the concrete volume and bag count on one screen.
Updated
Bars per direction are the clear span (dimension minus cover both sides) over the spacing, rounded up, plus one closing bar. Total rebar is every bar summed; a bar longer than one stock stick gets a 40-diameter lap added. Sticks is how many lengths to buy.
- Bars
- 22
- 20 ft sticks
- 12
- Weight (lb)
- 140
How the layout works
A slab or pad footing is reinforced with a mat: bars running each way, spaced across the clear span. The clear span is the dimension minus the concrete cover on both sides, and the bar count is that span divided by the spacing, rounded up so no gap is wider than you asked, plus one closing bar. Each bar's length is the clear dimension it crosses. A continuous footing runs longitudinal bars down its length instead, with optional ties. A pier runs vertical bars. Total rebar is every bar summed, and when a bar is longer than one stock length it gets a lap splice added.
A worked example
A 10 ft by 10 ft slab with #4 bar at 12 in on center, 3 in of cover, bought in 20 ft sticks. Here is the mat and the buy, and where each number comes from.
| Clear span | 9.5 ft | 10 ft less 2 x 3 in cover |
| Bars each way | 11 | ceil(9.5 / 1 ft) + 1 |
| Total bars | 22 bars | 11 each way, both directions |
| Total rebar | 209.0 ft | 22 bars x 9.5 ft |
| 20 ft sticks | 12 | 6 each way: 11 bars, 2 per stick |
| Weight | 140 lb | 209 ft x 0.668 lb/ft |
Slab, footing, or pier
The four shapes reinforce differently, which is why the calculator asks for the inputs it does. Match your pour to one of these.
| Shape | It reinforces with | Key input |
|---|---|---|
| Slab | An each-way mat of bars | Length, width, spacing |
| Pad footing | The same mat, times how many pads | Length, width, count |
| Continuous footing | Longitudinal bars, optional ties | Run, longitudinal bar count |
| Column or pier | Vertical bars per pier | Depth, vertical bars, count |
Bar sizes and weight
Rebar is called out by an eighth-of-an-inch number: a #4 bar is 4/8 in, or 1/2 in, in diameter. The weight comes from that size, and it is what sets the total pounds and the lap length. These are the US bar sizes the calculator carries.
| Bar | Diameter (in) | Weight (lb/ft) |
|---|---|---|
| #3 | 0.375 | 0.376 |
| #4 | 0.500 | 0.668 |
| #5 | 0.625 | 1.043 |
| #6 | 0.750 | 1.502 |
Cover, spacing, and laps
Cover is the concrete between the bar and the face, and it insets the bar on every side, so a 10 ft slab with 3 in of cover holds 9.5 ft of bar in each direction. Spacing is the maximum gap between bars; the count rounds up so the real gap never exceeds it. When a bar is longer than a stock length it is spliced, and the two bars overlap by a lap of about 40 bar diameters so the splice develops the bar. The calculator adds that lap to the total steel and to the stick count, which is why a long run buys more than a plain divide would suggest.
What the app adds
This page lays out the reinforcing steel. The app figures it next to the pour itself: the concrete volume in cubic yards, the bag count, the formwork contact area, and metric bar sizes, all on one footing screen. It prints the result as a material list you take to the yard.
Common questions
- How much rebar do I need for a slab?
- Count it as an each-way mat. A 10 ft by 10 ft slab with #4 bar at 12 in on center and 3 in of cover is 11 bars each way, 22 bars total, 9.5 ft long each, so 209 ft of rebar. In 20 ft sticks that is 12 sticks. Change the spacing to 16 in and the bar count drops; tighten it to 8 in and it climbs.
- What spacing should slab rebar be?
- Twelve to eighteen inches on center is typical for a residential slab, with the tighter spacing for heavier loads or a thinner slab. The calculator rounds the count up so the real gap never exceeds the number you enter: a 9.5 ft clear span at 12 in comes out to 11 bars, which is a real gap of about 11.4 in.
- How do I figure a lap splice?
- When a bar run is longer than a stock length, the two bars overlap so the splice can carry the load. A common lap is 40 bar diameters, so a #4 bar (1/2 in) laps about 20 in and a #5 bar about 25 in. The calculator adds the lap automatically once a run passes the stock length you pick, and counts the extra stick.
- How many bars for a footing?
- A continuous footing runs longitudinal bars down its length, usually two to four, plus ties or stirrups if the design calls for them. Enter the run, the bar count, and whether to include ties, and the calculator returns the total feet and the stick count. A pad footing is a small mat instead, and it multiplies by how many pads you pour.
- Does the total include the cover?
- The bar lengths are already cut back for cover, so the total is the steel you actually place, not the outside dimension. A 10 ft slab with 3 in of cover gives 9.5 ft bars, not 10 ft. The stick count is figured from those cut lengths, with a lap added only where a bar is spliced.
Lay out the steel, size the pour
CutOnce lays out the rebar like this page, then adds the concrete volume, the bag count, and the formwork, all on one footing screen on the jobsite.
Free to download, with a 7-day trial of everything. This page lays out the steel; the app prints the material list for the steel and the concrete together.
See also