Rebar Calculator: Spacing, Weight & Cost Guide

I Was Eyeballing Rebar Spacing and It Cost Me

I remember standing at the edge of a foundation form — maybe my third or fourth slab pour — and I was just kind of guessing where to put the rebar. Every 12 inches? Every 18? I honestly didn't know, and the guy helping me that day didn't either (or at least he pretended not to). We laid it out, poured the concrete, and about six months later there was a crack running diagonally across the whole thing. Not a hairline crack. A real one.

That's when I started actually doing the math.

The thing is, rebar isn't complicated once you sit down with it. But most people — contractors included, honestly — either over-order because they're nervous about running short, or they wing the spacing because "close enough" feels good in the moment. Neither approach is great for your wallet or your slab.

So yeah, I built a calculator for this. And I'm going to walk you through the logic behind it so you actually understand what's happening, not just blindly plugging in numbers.

How Rebar Spacing Actually Works (And Why 12" Isn't Always the Answer)

Rebar spacing is measured center-to-center. That means from the middle of one bar to the middle of the next one. Not edge to edge — center to center. I see people get this wrong all the time and it throws off everything downstream.

For a typical residential slab — say a garage floor or a patio — you'll often see #4 rebar (that's half-inch diameter) spaced at 12 inches on center, both directions. That gives you a grid pattern. But here's the thing: the spacing depends on what the slab is doing. A driveway that's going to have trucks on it? You might go 8 inches on center. A simple sidewalk? Maybe 16 or even 18 inches is fine. Your engineer or local code will tell you, and honestly you should listen to them on this one.

The formula for figuring out how many bars you need in one direction is pretty straightforward:

💡 THE FORMULA

Number of Bars = (Slab Length ÷ Spacing) + 1

Slab Length = the dimension perpendicular to the bars (in inches or feet, just be consistent)
Spacing = center-to-center distance between bars
The +1 accounts for the bar at each edge

Then you do the same thing for the other direction. So for a 20 ft × 30 ft slab with 12-inch spacing, you'd need:

Long direction: (30 ÷ 1) + 1 = 31 bars, each 20 ft long
Short direction: (20 ÷ 1) + 1 = 21 bars, each 30 ft long

That's 52 bars total. And you haven't even accounted for overlap yet — rebar comes in 20-foot lengths typically, so those 30-foot runs need splices. The standard lap splice is 40 times the bar diameter, which for #4 rebar works out to 20 inches of overlap. Some inspectors want 24 inches. I just go with 24 and call it a day.

You can use our rebar spacing calculator to skip the manual math, but knowing the logic means you can sanity-check whatever number pops out.

Rebar Weight and Cost — The Numbers Nobody Talks About Until the Invoice Shows Up

Rebar is sold by weight or by length, depending on your supplier. Either way, you need to know the weight per foot for whatever size you're using. Here's a table I keep bookmarked in my brain at this point:

So let's go back to our 20×30 slab example with #4 rebar. We calculated 52 bars. But we need to account for the actual total linear footage:

31 bars × 20 ft = 620 ft
21 bars × 30 ft = 630 ft (plus splice material — roughly another 40-50 ft for overlaps)
Total: about 1,290 to 1,300 linear feet

At 0.668 lbs per foot, that's around 868 lbs of rebar. Give or take. And at typical pricing of somewhere in the ballpark of 0.50 to 0.85 per foot (prices fluctuate like crazy, so check locally), you're looking at maybe 650 to 1,100 for the rebar alone on a pretty modest slab. That's before labor, chairs, tie wire, and all the other little stuff that adds up.

I didn't believe how fast it added up the first time I did a real takeoff.

If you're doing a bigger project — like a commercial foundation or a retaining wall — the numbers get wild. A concrete volume calculator paired with a rebar calculator will give you the full picture of material costs. And if you're trying to figure out what your total project budget looks like, our concrete cost estimator is worth a look too.

Quick Tips From Too Many Pours

Always add 10-15% waste to your rebar order. Cuts, bends, mistakes — it happens. I'd rather return a few sticks than make a second trip to the yard.

Rebar chairs matter more than people think. If your rebar is sitting on the ground instead of suspended in the middle of the slab, it's basically decorative. I've seen guys pour over rebar that sank to the bottom and then wonder why the slab cracked. The rebar needs to be in the tension zone — usually the lower third for slabs on grade.

Tie wire: buy more than you think you need. It's cheap and you'll use it.

For area calculations on irregular shapes (L-shaped foundations, curved walls, whatever), the area calculator can help you break things into manageable pieces. And if you're converting between units — because half your measurements are in feet and the other half are in inches for some reason — the unit converter will save you from dumb mistakes.

One more thing: if you're estimating labor costs and trying to figure out what you're paying your crew per hour versus what you quoted, our salary to hourly converter is surprisingly useful for that kind of back-of-envelope math.

FAQ

How much rebar do I need per square foot of slab?

It depends entirely on your spacing. At 12" on center both ways with #4 bar, you're looking at roughly 2 linear feet of rebar per square foot of slab (1 foot in each direction, plus a little extra for overlaps). For a 600 sq ft slab, that's about 1,200 linear feet — which tracks with the example I worked through above. Use our rebar calculator to get an exact number for your specific dimensions and spacing.

What's the difference between #4 and #5 rebar?

#4 is half-inch diameter, weighs 0.668 lbs/ft. #5 is five-eighths inch, weighs 1.043 lbs/ft. The jump in weight (and cost) is significant. #5 is stronger but harder to bend by hand and overkill for most residential slabs. Your structural plans will specify which one — don't just upgrade because it "feels" stronger.

Can I use wire mesh instead of rebar?

For light-duty slabs like patios and shed floors, welded wire mesh (6×6 W1.4/W1.4 is common) can work. But it's not a substitute for rebar in structural applications — foundations, retaining walls, anything carrying real load. Mesh also has a tendency to end up on the ground during a pour if you're not careful with supports. I personally prefer rebar for anything I actually care about lasting, but that's me. Check your local code and talk to your engineer.