Rebar Calculator
Enter your slab size, bar spacing and bar size to get the number of bars each way, the total linear length, the reinforcement steel weight and the cost.
Use this free rebar calculator to plan the reinforcing steel for a slab laid out as a grid. Enter the slab length and width, the bar spacing on centre, the edge clearance and the bar size, and it works out how many bars run each way and their total length. As a rebar weight calculator it converts that length into pounds and kilograms using the nominal bar unit weight, and it prices the job from your cost per foot. Add a stock length and lap and it counts the overlap where a bar run is longer than one stick.
Slab & bars
Total rebar length
385 ft
257.18 lb · 116.65 kg of #4 steel
- Bars lengthwise
- 10
- Bars widthwise
- 20
- Total weight
- 257.18 lb
- Material cost
- —
Bars in each direction are placed from the spacing, kept back from the edges by the clearance. Weight uses the nominal unit weight for the bar size, so it is the figure a supplier quotes tonnage from.
A planning estimate for a flat rectangular slab with a single mat. Chairs, corner bars, extra top steel and cut waste all add material, and lap lengths are code-driven, so confirm the schedule against your engineer's drawings before ordering.
How it works
- 1
Enter the slab
Add the slab length and width in feet or metres. The grid is figured for a flat rectangular pour.
- 2
Set the grid
Type the bar spacing on centre and the edge clearance in inches, then pick the bar size from #3 to #8 or a metric bar.
- 3
Read the steel
See the bar count each way, the total linear length, the weight in pounds and kilograms, and the cost.
Instant & 100% private — nothing is uploaded
Every calculation runs locally in your browser. The income, balances and goals you enter stay on your own device and are never sent to a server — nothing is stored, logged or shared.
Frequently asked questions
- How do I calculate how much rebar I need?
- Divide the slab width by the bar spacing and add one for the bar count in one direction, then do the same across the length for the other. Multiply each count by the bar run length and add the two totals. The calculator does this from your spacing and edge clearance.
- How is the rebar weight worked out?
- Each bar size has a nominal weight per foot: a #4 is 0.668 lb/ft, a #5 is 1.043, a #8 is 2.670. The total linear length times that unit weight gives the reinforcement steel weight, shown here in both pounds and kilograms.
- What bar spacing should I use for a slab?
- Residential slabs are commonly reinforced on a 12 to 18 inch grid, tightening to 12 inches or less for heavier loads. The spacing and bar size come from the structural design, so follow your engineer's drawing rather than a rule of thumb.
- Does it account for lap splices?
- Yes, if you enter a stock bar length and a lap length. Where a bar run is longer than one stick, it adds the overlap for each splice so the rebar quantity reflects the extra steel. Leave the lap at zero to skip it.
- What is edge clearance?
- It is the cover of concrete kept clear between the bars and the slab edge, usually 2 to 3 inches so the steel does not sit at the surface. The calculator keeps the outer bars back by that amount when it spaces the grid.
- How accurate is the rebar estimate?
- It is a close planning figure for a single flat mat on a rectangular slab. Corner bars, a second top mat, chairs and cutting waste all add steel, and lap lengths are code-driven, so confirm the final bar schedule against the engineered drawings before you order.
More tools
More from the Hivly network
Free sister tools on our other sites.
