Rebar Spacing & Weight Calculator

Rebar Bar Size Reference Chart

How to Use This Rebar Calculator

Choose Slab / Footing, Wall or Circular Slab, then enter the dimensions in feet and the rebar spacing in inches, measured center to center. Pick the bar size, the stock bar length you plan to buy, the lap splice length and a waste allowance. The calculator returns the bar count in each direction, total length including splices, the number of stock bars to buy and the steel weight, and it redraws the diagram with your numbers.

Why This Matters

Rebar is bought by the stick and priced by the pound, so a bad count costs you twice: once at the supplier and again on pour day. Take a 20 × 20 ft driveway or shop slab with #4 bars at 12 in. on center. That is 21 bars each way, 42 bars in total, 840 linear feet and about 561 lb of steel. Add 10% for cuts and waste and you order 47 twenty-foot bars weighing roughly 628 lb, which is a real load for a pickup. A 30 ft × 8 ft wall with the same bar at 12 in. each way needs 9 horizontal and 31 vertical bars: about 533 ft with splices and 356 lb before waste, or 30 sticks (401 lb) once waste is included. Contractors use this for takeoffs, homeowners for patios and garage pads, and estimators to compare spacing before bidding. Tightening the slab above from 12 in. to 8 in. raises the steel by nearly 50%, so it pays to see the difference before you buy.

How It's Calculated

Bars in one direction: bars = ceil(dimension_ft × 12 ÷ spacing_in) + 1. Rounding up means the actual spacing is never wider than the value you entered.

Splices: if a run is longer than the stock bar, each extra piece overlaps the previous one by the lap length: pieces = ceil((run − lap) ÷ (stock − lap)) and run length with splices = run + (pieces − 1) × lap.

Order length and bars to buy: order_ft = total_ft × (1 + waste%), then bars_to_buy = ceil(order_ft ÷ stock_length).

Weight: weight = length × lb_per_ft using ASTM A615 nominal weights. The tool shows the calculated weight and the heavier weight of the whole sticks you buy.

Circular slabs: each bar is a chord of the circle, chord = 2 × √(r² − d²), where r is the radius and d is the bar's distance from the center. Bars run in two directions.

Spacing and lap values are rules of thumb in the spirit of ACI 318, which limits slab bar spacing to the lesser of 3× the slab thickness or 18 in., and lapping bars about 40 diameters for Grade 60 steel. Top bars, epoxy-coated bars and seismic zones need longer laps. Local codes vary.

Worked Example

A 20 × 20 ft slab, #4 bars at 12 in. on center, 20 ft stock bars, 20 in. lap, 10% waste (the tool's defaults):

1. Bars each way: ceil(20 × 12 ÷ 12) + 1 = 21.
2. Each run is 20 ft, which fits in one 20 ft stock bar, so there are no splices.
3. Total length: 2 directions × 21 bars × 20 ft = 840 ft.
4. Calculated weight: 840 × 0.668 = 561.1 lb.
5. With 10% waste: 840 × 1.10 = 924 ft.
6. Bars to buy: 924 ÷ 20 = 46.2, rounded up to 47 bars.
7. Order weight: 47 × 20 × 0.668 = 627.9 lb.

Tips & Common Mistakes

• Follow the plans first. If your drawings call for a bar size and spacing, use those. Spacing wider than 18 in. in a slab usually exceeds common limits.
• Don't skip the lap. Splices add length, and short laps weaken the connection. Wire-tie every lap and check your plan's required length.
• Plan for cover and chairs. Bars sit about 3 in. off the soil in a slab on ground and stop short of the edges. This calculator uses full dimensions, which slightly overestimates length on purpose.
• Add extras separately. Corner bars, dowels, bends, hooks and bars around openings are not included in the grid count.
• Check the haul. A few hundred bars can weigh over a ton. Know the order weight before you pick a trailer or delivery truck.

Frequently Asked Questions

What rebar size and spacing should I use for a concrete slab?

Residential slabs on ground often use #3 or #4 bars at 12–18 in. on center, but the right answer depends on slab thickness, soil and loads. Heavier uses such as RV pads or vehicle parking may call for closer spacing or larger bars. Always confirm with your plans or local building department.

What does rebar spacing mean?

Spacing is the center-to-center distance between parallel bars. A 12 in. spacing means each bar's centerline is 12 in. from the next. Closer spacing adds strength and crack control but also adds steel cost and tie time.

How long are standard rebar sticks?

Home centers commonly sell 10 ft and 20 ft bars, and steel distributors stock 40 ft and 60 ft lengths. Enter whatever length you will actually buy, and the calculator adjusts splices and the number of bars.

How much does rebar weigh per foot?

#3 weighs 0.376 lb/ft, #4 weighs 0.668 lb/ft and #5 weighs 1.043 lb/ft. A 20 ft #4 bar therefore weighs about 13.4 lb. The chart above lists every common size.

How much waste should I add?

Five percent is a minimum on simple rectangular layouts, and 10% is safer for walls, footings with corners or layouts with many cuts. Running a little long is cheaper than stopping a pour to fetch more steel.

Does this tell me if my rebar design is adequate?

No. It counts and weighs steel for a layout you choose. Structural adequacy depends on loads, soil, concrete strength and local code, and has to come from your plans or a licensed engineer.