How Much Rebar for a 30x40 Slab?
Rebar to buy
83 sticks
1,200 sq ft, #4 bar at 18 in both ways, 3 in cover, 20 ft sticks, 40 diameter lap splices.
When the figures look right, the rebar material list downloads as a real PDF with your business details on it, built inside your own browser. Nothing you type is uploaded anywhere.
How that number is worked out
These are the figures this page was built with and every line the calculator produces from them. Change anything above and all of it moves.
- Slab length30 ft
- Slab width40 ft
- Bar spacing, both ways18 in
- Bar size#4, 1/2 in
- Cover from the edge3 in
- Bar stock length20 ft
- Lap splice40 bar diameters
Rebar to buy83 sticks
| Slab area | 1,200 sq ft |
|---|---|
| Bars running the length | 2729.5 ft each, spaced across the width |
| Bars running the width | 2039.5 ft each, spaced along the length |
| Linear feet, no laps | 1,586.5 ft |
| Lap allowance | 60 ft36 splices at 1.67 ft each |
| Total linear feet | 1,646.5 ft |
| Sticks at 20 ft | 831,660 ft bought, 36 splices, offcuts carried into the next run |
| Weight | 1,100 lb0.55 tons |
| Intersections to tie | 540 |
| Chairs | 300one per 4 sq ft |
The material list this produces
| #4 rebar, 20 ft sticks | 83 |
|---|---|
| Tie wire, lb | 4 |
| Chairs | 300 |
| Poly vapor barrier, sq ft | 1,320 |
Bar stock length compared
Stock length swept with everything else held. The middle column is the lap allowance, and it should be read against the first: short stock buys you more sticks and more lap at the same time. At 10 ft stock, 225 of the feet you pay for are lap.
| Bar stock length | Rebar to buy | Lap allowance | Total linear feet |
|---|---|---|---|
| 10 ft sticks | 182 sticks | 225 ft | 1,811.5 ft |
| 20 ft sticks | 83 sticks | 60 ft | 1,646.5 ft |
| 30 ft sticks | 54 sticks | 11.7 ft | 1,598.2 ft |
| 40 ft sticks | 47 sticks | 0 ft | 1,586.5 ft |
| 60 ft sticks | 34 sticks | 0 ft | 1,586.5 ft |
What else matters here
Twelve hundred square feet, 1,100 lb of steel and 83 sticks. At this size the biggest lever on the order is not the spacing and it is not the bar size. It is the length of the stick you buy, and almost nobody asks the yard about it.
The table below sweeps stock length and holds everything else still. On 20 ft sticks this slab needs 83 of them and carries 60 ft of lap across 36 splices. On 40 ft sticks it needs 47 and has no splices at all, because a 39.5 ft run fits a 40 ft bar whole. On 10 ft sticks it needs 182, and 225 ft of the steel you buy is lap, which is steel you pay for and place and which does nothing except hold two bars together.
Long bar is a delivery question rather than a price question. Twenty foot stock rides on a pickup with a flag on the end and 40 ft stock does not, so on a pour this size the choice usually comes down to whether the yard will drop a bundle on the site. If they will, the order falls by more than a third and the tying gets faster too, because 36 splices are 36 places where somebody has to get down on the ground.
What this calculator assumes
- Bars are figured as a grid running both ways at the same spacing, which is how a residential slab, a driveway and a footing mat are normally drawn.
- Weights are the published ones: #3 is 0.376 lb per foot, #4 is 0.668, #5 is 1.043, #6 is 1.502, #7 is 2.044 and #8 is 2.670.
- The stick count is nested, not divided. When a run fits inside a stock bar, the count says how many runs are cut from one stick and never splices a bar that did not need splicing. When a run is longer than a stock bar it is built from a chain of sticks with a lap at every joint, and the offcut from one run starts the next. That is why the stick line and the linear feet line never agree, and the gap between them is the offcut you pay for.
- A 40 diameter lap is the common residential rule of thumb. ACI 318 sets the real development length from the concrete strength, the bar position and the cover, and an engineered drawing overrides anything on this page.
- Chairs are one per four square feet, which keeps the mat off the ground on an ordinary pour. Wet setting bar into poured concrete is a different method and this list does not price it.
Questions people ask
How much rebar for a 30x40 slab?
27 bars one way and 20 the other at 18 in spacing with #4 bar, 1,646.5 ft including the laps, 83 sticks of 20 ft steel and 1,100 lb.
Does buying longer rebar save money on a 30x40 slab?
It saves steel. On 20 ft sticks the slab takes 83 and carries 60 ft of lap. On 40 ft sticks it takes 47 and carries none, because a 39.5 ft run fits a 40 ft bar whole.
Does a bigger bar size cost more sticks?
Almost none. Going from #3 to #8 on this slab moves the stick count from 82 to 87, while the weight goes from 613 lb to 4,592. Bar size is a weight and price decision, not a quantity one.
The rebar calculator takes any dimensions you like and hands back the same list.