How do I calculate how much rebar I need for a slab?

Subtract twice the edge cover from each slab dimension to get the clear spans. Divide each clear span by the bar spacing, take the whole number, and add one — that is the bar count in each direction. Multiply each count by the length of those bars, add the two together, then add lap steel for any run longer than a single stock length, and finally add a wastage allowance. The step people miss is the plus one.

Why is it bars plus one and not just span divided by spacing?

Because you are counting fenceposts, not fence panels. A run of fence with four panels has five posts, one at each end of every panel. Bars work the same way: a 10 ft clear span at 12 inch centres has 10 gaps between bars, and each gap has a bar at both ends, giving 11 bars. Dividing and stopping counts the gaps rather than the bars. On a typical slab this error costs you one bar in each direction, which is easy to absorb on a small job and expensive on a large one.

What spacing should I use for rebar in a slab?

For a residential slab on grade, 12 to 18 inch centres with #3 or #4 bar covers most cases. Driveways and patios often use #4 at 16 or 18 inches; a garage floor carrying vehicle loads more commonly uses #4 at 12. Going tighter than about 4 inches starts to cause problems, because aggregate needs room to pass between the bars for the concrete to consolidate properly around the mat. Wider than 18 inches and the steel does progressively less for crack control, with many codes setting 18 or 24 inches as an outright maximum.

What is a lap splice and how long should it be?

A lap splice is the overlap where two bars are joined. They are not welded or butted; the two bars simply run alongside each other for a distance, and the load transfers between them through the surrounding concrete. Forty bar diameters is a common figure for a tension lap in ordinary work, so a #4 bar laps 20 inches and a #5 laps 25. The actual required length depends on concrete strength, bar grade, spacing, cover and whether the bar is coated, which is why the structural drawings specify it rather than a rule of thumb.

How much cover does rebar need?

Cover is what protects the steel from moisture and keeps it from corroding, and it is not optional. For concrete cast against earth the usual minimum is 3 inches. For concrete exposed to weather but formed, it drops to 2 inches for larger bars and 1½ inches for #5 and smaller. For interior slabs not exposed to weather or ground it can be as little as ¾ inch. Rust occupies several times the volume of the steel it came from, so a bar that is too close to the surface will eventually crack the concrete off from the inside.

Where in the slab thickness should the mat sit?

For a single mat in a slab on grade, roughly the middle to lower third, supported on chairs or bar supports so it stays there during the pour. The single most common mistake in DIY concrete is laying the mat on the ground and expecting to pull it up as the concrete goes in. Steel resting on the base does nothing at all — it is below the zone where the slab is in tension, and it is in permanent contact with moisture. If the slab needs top and bottom steel, that is two mats, which is what the layers setting is for.

Do I need to tie every intersection?

No. Tying every crossing is normal for a structural mat but unnecessary for a slab on grade, where alternate intersections is standard practice and perfectly adequate. The ties do not add strength — their only job is to stop the mat shifting while the concrete is placed. Around 18 inches of 16 gauge wire per tie is a reasonable allowance once you account for the twist and the offcut, which works out to roughly 220 ties from a standard 3.5 lb roll.

Can I use welded wire mesh instead of rebar?

For light residential slabs, often yes, and it is quicker to place. Mesh is well suited to crack control in a slab on grade with no significant structural demand. Where it falls down is anything carrying real load or spanning ground that may move, since the wire is much lighter than even a #3 bar and the sheets need proper lapping to act continuously. Mesh is also notoriously prone to being walked flat during the pour, which leaves it sitting on the base doing nothing — the same failure as an unsupported rebar mat, and just as common.

How much waste should I allow on a rebar order?

Ten percent is a sensible default and it is what this calculator uses. Rebar waste is mostly offcuts: cutting a 29.5 ft run from 20 ft lengths leaves a 9.5 ft remnant that may or may not be usable elsewhere on the job. Slabs with simple rectangular geometry and long runs waste less, because the offcuts are big enough to reuse. Complex shapes with many short bars waste more, since every cut leaves a piece too short for anything. Ordering a little over is cheap; a second delivery for four bars is not.

Rebar Quantity Calculator

Working out how much reinforcing bar a slab needs looks like simple division and it hides two traps that between them will leave you short on almost every job. The first is a counting error. Bars laid across a span are gaps plus one, not span divided by spacing — a twenty foot clear span at twelve i...

THE MAT

SLAB LENGTH (ft)

SLAB WIDTH (ft)

BAR SPACING (ft)

EDGE COVER (ft)

BAR SIZE

0.500 in diameter · 0.994 kg/m · 0.668 lb/ft

REBAR TO ORDER

67 lengths

#4 bar · 20 ft each · 1,329.2 ft of bar in total

LENGTHWISE

20 bars

CROSSWISE

30 bars

WEIGHT

403 kg

Lengthwise bars

20 × 29.5 ft

Crosswise bars

30 × 19.5 ft

Bar before laps

1,175.0 ft

Lap joints

20 × 1.67 ft

Bar with laps

1,208.3 ft

With 10% waste

1,329.2 ft

Total weight

402.9 kg / 888 lb

Intersections to tie

600

SETTING THE MAT

Order 67 lengths of #4 at 20 ft, about 403 kg of steel.

Cut 20 bars to 29.5 ft and 30 to 19.5 ft. Bars stop at the cover line, never at the slab edge — steel that reaches the edge rusts and spalls the concrete off.

Lay the mat on chairs so it sits within the slab rather than on the ground. Steel resting on the base carries nothing and corrodes.

Tie 600 intersections — roughly 4.3 kg of 16 gauge wire at 18 in per tie. Every intersection does not need tying; alternate ones is normal practice for a slab mat.

THE MAT

Bars are gaps plus one. The clear span across the width is 19.5 ft, which at 1 ft centres gives 19 gaps — and 20 bars, because there is a bar at each end of every gap. Dividing the span by the spacing and stopping there is the classic fencepost error, and it leaves you one bar short in each direction on every job.

Live calculation · updates as you type

Created with❤️byeaglecalculator.com

HOW TO USE

1
Enter the slab length and width, then the bar spacing and the edge cover. Cover is the distance from the slab edge to the first bar — steel that runs to the edge will rust and spall the concrete off, so bars always stop short.
2
Choose your bar size. US sizes are eighths of an inch, so a #4 is four eighths, or half an inch. Metric sizes are the diameter in millimetres. The weight per unit length appears underneath as you change it.
3
Open site adjustments to set the stock length your supplier sells, the lap as a multiple of bar diameter, and how many mats you are laying. Two mats means top and bottom steel, which doubles everything.
4
Read off the number of lengths to order. The breakdown shows bar counts each way, the linear total before and after laps, the total weight, and how many intersections need tying.

THE FORMULAS

Bars in one directionfloor(clear span / spacing) + 1

Clear spanslab dimension − 2 × cover

Bar lengthother dimension − 2 × cover

Pieces per run1 + ceil((run − stock)/(stock − lap))

Lap lengthmultiplier × bar diameter

Total bar(bars × length + laps) × (1 + waste)

Bar massπ/4 × d² × 7850 kg/m³

Intersectionsbars one way × bars the other

BAR SIZE REFERENCE

Bar	Diameter	kg/m	lb/ft	40d lap
#3	0.375 in	0.559	0.376	15.0 in
#4	0.500 in	0.994	0.668	20.0 in
#5	0.625 in	1.554	1.044	25.0 in
#6	0.750 in	2.237	1.503	30.0 in
#7	0.875 in	3.045	2.046	35.0 in
#8	1.000 in	3.978	2.673	40.0 in
#9	1.128 in	5.061	3.401	45.1 in
#10	1.270 in	6.416	4.311	50.8 in
#11	1.410 in	7.908	5.314	56.4 in
8mm	8 mm	0.395	0.265	320 mm
10mm	10 mm	0.617	0.414	400 mm
12mm	12 mm	0.888	0.597	480 mm
16mm	16 mm	1.578	1.061	640 mm
20mm	20 mm	2.466	1.657	800 mm
25mm	25 mm	3.853	2.589	1,000 mm

Weights are computed from the nominal diameter at a steel density of 7,850 kg/m³ rather than taken from a table, so the imperial and metric figures cannot disagree with each other. Every value matches published mill data to within 0.2%.

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Last updated: July 28, 2026 · Formula verified · Eagle-eyed accuracy for every calculation.