Rebar Spacing Calculator

How the rebar calculator works

The rebar calculator computes the total linear feet of #4 bar needed for a slab, given the slab dimensions and the grid spacing. The formula subtracts 6 inches (3-inch edge cover on each side) from both the length and width, then divides each by the spacing to get the number of bars in each direction. Total linear feet = (bars across × length) + (bars along × width).

A worked example: a 10×10 slab at 24-inch spacing. Bars across (along the width): floor((10 − 0.5) ÷ (24/12)) + 1 = floor(9.5 ÷ 2) + 1 = 4 + 1 = 5 bars. Each bar is 10 feet long. Bars along (along the length): same calculation, 5 bars each 10 feet long. Total linear feet = (5 × 10) + (5 × 10) = 100 linear feet of #4 bar. At 0.668 lb/ft, that's 67 pounds of steel — about $50 worth.

The 3-inch edge cover follows ACI 318 §20.6.1 for concrete cast against and permanently exposed to earth — it keeps the rebar far enough from the edge that it doesn't corrode from soil moisture. For slabs exposed to weather or deicing salts, ACI 318 §20.6.1 requires 2 inches minimum. For slabs in non-corrosive environments (interior, dry), 1.5 inches is acceptable. The calculator uses 3 inches as the default; adjust based on your application.

Rebar sizes, grades, and what to use when

Rebar comes in sizes numbered #3 through #18, where the number is the bar diameter in eighths of an inch. #3 bar is 3/8 inch diameter, #4 is 1/2 inch, #5 is 5/8 inch. For residential slabs 4–6 inches thick, #4 is the standard — strong enough to provide crack control, cheap enough to be economical, and small enough to be easily handled. #3 is too small for slabs (used for walls and footings), and #5 is overkill for most residential work.

Rebar grades refer to the steel's minimum yield strength, in thousands of psi. Grade 40 (40,000 psi) is the lowest available and is being phased out. Grade 60 (60,000 psi) is the modern standard and is what you'll find at big-box stores. Grade 75 and Grade 80 are used in commercial construction. For residential slabs, buy Grade 60 — it's the same price as Grade 40 and significantly stronger.

Epoxy-coated rebar is used in corrosive environments — coastal areas, slabs exposed to deicing salts, or anywhere with high chloride exposure. The epoxy coating prevents corrosion from chloride ions, which can eat through uncoated rebar in 10–15 years. Epoxy-coated rebar costs 30–50% more than plain rebar; use it for driveways in cold climates or anywhere salt will be applied.

For most residential slabs, skip the rebar entirely and use welded wire mesh (WWM) instead. WWM at 6×6 W1.4×W1.4 (6-inch grid, 1.4 gauge wire) provides crack control equivalent to #3 bar at 12-inch centers, at one-third the cost. The downside is that WWM is hard to keep in the right position during a pour — workers tend to step on it and push it to the bottom of the slab, where it does nothing.

Placement, tying, and what the rebar estimate excludes

Rebar placement is critical: it must be in the bottom third of the slab (1.5–2 inches from the bottom for a 4-inch slab) to resist tension. Concrete is strong in compression (resists pushing from above) and weak in tension (resists bending from below). Rebar's job is to hold the bottom of the slab together when it tries to bend, so it belongs in the bottom third. Rebar in the middle of a slab wastes 50% of its strength; rebar on top is useless.

Rebar chairs (plastic or steel supports that hold the rebar at the right height during the pour) are the most-overlooked accessory. Use 1.5-inch chairs for a 4-inch slab (gives 1.5-inch cover top and bottom), 2-inch chairs for a 5-inch slab. Place one chair per 4 square feet of slab — about 25 chairs for a 10×10 slab, 100 chairs for a 20×20. At $0.10 each, this is a $2.50–10 cost that's easy to forget.

Tying the rebar intersections keeps the grid in place during the pour. Use 16-gauge annealed tie wire (a 3.5-lb roll at $15 ties about 1000 intersections — enough for most residential jobs). Use a tie-wire twister ($8) to spin the wire quickly. Tie every intersection — loose intersections let the grid shift when workers step on it. Rebar ties should be tight enough that the bars don't move but loose enough that the wire doesn't break.

What the rebar calculator excludes: rebar chairs ($0.10 each, 1 per 4 sf), tie wire ($15 per 3.5 lb roll, 1 per 1000 ties), a tie twister ($8), bolt cutters or abrasive saw to cut #4 bar ($30 for a manual cutter, $80 for an abrasive saw), and labour (figure 30 minutes per 100 square feet to lay and tie a #4 grid). The rebar itself runs $0.50–0.80 per linear foot for #4 Grade 60.

For welded wire mesh (the cheaper alternative): 6×6 W1.4×W1.4 sheets come in 8×15 foot rolls at $30 per roll (covers 120 sf, $0.25/sf). WWM must be lifted into the top third of the slab during the pour — use a hook tool ($8) to pull it up as the concrete is placed. WWM is faster to install than rebar (no tying) but provides less strength. For driveways and garage floors, use rebar; for patios and walkways, WWM is sufficient.

Common mistakes and how to avoid them

The most common rebar mistake is incorrect placement within the slab. Rebar belongs in the bottom third of the slab — 1.5 to 2 inches from the bottom for a 4-inch slab. Concrete is strong in compression (resists pushing from above) and weak in tension (resists bending from below). Rebar resists tension, so it belongs where the tension is: the bottom. Rebar in the middle of a slab wastes 50% of its strength; rebar on top is useless. Use rebar chairs to hold the bars at the correct height during the pour — workers will step on the grid and push it to the bottom if it is not supported.

A second mistake is using the wrong bar size. #3 bar (3/8 inch) is too small for slabs — it is used for walls and footings. #4 bar (1/2 inch) is the standard for residential slabs 4-6 inches thick. #5 bar (5/8 inch) is for driveways, garage floors, or anywhere with heavy loads. Using #3 instead of #4 to save $0.20 per linear foot on a 1000-sf slab saves $20 and reduces crack resistance by 40%. Always use #4 or larger for slabs.

Edge cover is frequently wrong. ACI 318 requires 3 inches of cover for concrete cast against earth (slabs on grade) — meaning the rebar must be at least 3 inches from the edge of the slab. For slabs exposed to weather, 2 inches minimum. For interior dry slabs, 1.5 inches. The calculator uses 3 inches as the default. If your edge cover is less than code, the rebar will corrode from soil moisture and stain the concrete surface within a few years.

Tying is the fourth common error. Every intersection should be tied with 16-gauge tie wire — loose intersections let the grid shift when workers step on it. A tie-wire twister ($8) makes this fast: spin the wire 3-4 times per intersection. Do not over-tighten — the wire should be snug enough to hold the bars together but not so tight that it cuts into the steel. A 3.5-lb roll of tie wire ($15) ties about 1000 intersections, enough for most residential slabs.

Finally, do not forget the rebar chairs. One chair per 4 square feet of slab — 25 chairs for a 10x10, 100 chairs for a 20x20. At $0.10 each, this is a $2.50-10 cost that is easy to forget. Without chairs, the rebar grid ends up on the subgrade, where it provides no reinforcement. Set the chairs first, lay the rebar on top, tie the intersections, and check the height with a tape measure before the pour. A 1.5-inch chair gives 1.5 inches of cover top and bottom on a 4-inch slab — the correct position.