Rebar Calculator: How Much Rebar to Order for Slabs
A rebar calculator turns slab size into stick count, splice allowance, and weight; for most flatwork, 2, 3, or 4 rebar is the common order range. Buying too little leaves short bars and weak laps, which can mean a cracked slab and a lost pour day. We show the formula, stick-length planning, weight by bar size, and a quick rule for when rebar beats mesh or fibers.
What this rebar calculator does
Inputs it uses: length, width, spacing, cover, and bar size
The inputs are length, width, spacing, cover, and rebar size. For slab grids, we count bars in both directions. For footings and walls, we count straight runs, laps, and turns instead of a two-way mat.
What it returns: linear feet, stick count, waste allowance, and weight
The output should give more than a bar count. A useful takeoff shows total linear feet, how many 20 ft sticks to buy, how much waste to allow for laps and cutoffs, and the estimated weight. That gives an order quantity you can use.
When the result is a buy range instead of a single number
For rebar, the clean answer is often a range. Openings, edge cover, lap splices, non-rectangular cuts, and the choice of 20 ft versus 40 ft stock all change the order. A buy range helps avoid shortages without leaving a pile of short pieces.
How to calculate rebar for a slab
For a rectangular slab, count the bars in one direction by using the other direction’s clear width, then repeat for the perpendicular direction. Add edge cover to shrink the clear layout size. The total linear feet is the sum of all runs, then add lap waste and cutoffs before converting to sticks.
Grid math: bars in both directions
Use this basic layout formula for each direction: clear dimension divided by spacing, rounded up, then add one bar if needed to keep the outer bars inside cover. Multiply each bar count by the run length. Do that once for lengthwise bars and once for widthwise bars.
How edge clearance changes the count
Edge cover reduces the usable size of the grid. If a slab is 20 ft by 12 ft and cover is 2 in each side, the clear dimensions are slightly smaller than the full pour size, which can drop a bar or shorten each run. Small cover changes matter on tight layouts.
Worked example for a patio slab
Take a 20 ft by 12 ft patio with #4 rebar at about 12 in on center and roughly 2 in edge cover. Clear length is about 19 ft 8 in and clear width is about 11 ft 8 in. That yields roughly 12 bars running the 20 ft direction and 20 bars running the 12 ft direction, or about 424 linear feet before waste.
At 20 ft stock lengths, 424 linear feet is about 21 sticks. Add a practical 10% for laps, cuts, and field fit-up, and the buy range becomes 24 to 25 sticks. At #4 weight, that is about 284 to 297 lb, before local waste from trims and splices.
What rebar size should I use?
For residential and light commercial work, #3 through #8 are commonly used imperial sizes. The number system is in eighths of an inch, and some calculators also show metric bar sizes such as No. 10, 12, 16, 20, 25, 28, 32, 36, 40, 50, and 60. Size choice should match load and member thickness.
Rebar size numbers from #3 to #8
#3 is smaller and lighter, while #8 is much heavier and used for more demanding structural work. #4 is 1/2 inch in diameter and is common in slabs, footings, and some walls. #5 is a frequent upgrade when loads rise or when a heavier grid is desired.
Diameter in eighths of an inch and common metric equivalents
Imperial bar sizes #2 through #8 are sized in eighths of an inch. That means the number generally tracks bar diameter. Metric sizing is different, so a calculator should label units clearly rather than mixing them. If a spec calls for metric bar numbers, convert before ordering stock.
When smaller bars fit residential work and when heavier bars are more appropriate
Smaller bars can fit light slabs, narrow footings, and noncritical flatwork. Heavier bars suit thicker slabs, higher wheel loads, and more demanding structural members. As a practical rule, tighter spacing with smaller bars often works better than wide spacing with oversized bars, up to the limits of the design and code.
How much spacing and cover does a slab need?
Spacing and cover depend on the member, exposure, and structural demand. ACI 318 gives maximum slab spacing as the lesser of 3 times slab thickness or 18 inches. Common residential slab layouts often land between 18 and 24 inches on center, but structural slabs may need tighter spacing.
Typical 12 to 24 inches on center spacing
Residential flatwork commonly uses spacing in the roughly 12 to 24 inch range on center. A 4 inch slab has a maximum spacing of 12 inches on center under the ACI 318 spacing rule. Wider spacing can be fine for some nonstructural work, but it should not be assumed for every pour.
ACI 318 spacing guidance for structural concrete
For structural concrete, use the lesser of 3 times slab thickness or 18 inches. That rule is a quick screening check, not a substitute for project-specific design. If the member carries walls, vehicles, or point loads, the spacing often tightens and the bar size may increase.
Concrete cover rules for earth, formed surfaces, and slabs on grade
Cover protects steel and keeps it in the right location. Common guidance includes 3 inches against earth, 2 inches for formed surfaces exposed to weather, and 1.5 inches for slabs on grade with a vapor barrier. Cover choice changes the clear grid size and therefore the bar count.
How do I turn linear feet into sticks and weight?
Convert linear feet into sticks by dividing by stock length, then round up and add a waste allowance for laps and cutoffs. Standard rebar lengths are 20 feet, with 40-foot and 60-foot lengths also available. Weight comes from linear feet multiplied by pounds per foot for the chosen bar size.
20-foot stock length planning
Most buyers see 20 ft sticks first. Longer 40 ft and 60 ft lengths exist, but they are not always the simplest buy for a small order. A 20 ft stock plan is easier to cut into shorter runs, especially for patios, footings, and walls with corners.
Lap splice length and waste allowance
The standard lap splice length is 40 times the bar diameter. That means about 20 inches for #4, 25 inches for #5, and 30 inches for #6. When bars are spliced, the overlap adds weight and reduces usable length, so a takeoff should include it before ordering.
Weight conversion using published pounds-per-foot values
For ordering, use the listed weights per foot for each rebar size. Multiply total linear feet by the weight per foot to estimate total pounds. That gives a useful check on freight, handling, and cost.
| Bar size | Diameter | Weight per foot | Typical lap splice | 20 ft stick weight |
|---|---|---|---|---|
| #3 | 3/8 in | 0.376 lb/ft | 15 in | 7.52 lb |
| #4 | 1/2 in | 0.668 lb/ft | 20 in | 13.36 lb |
| #5 | 5/8 in | 1.043 lb/ft | 25 in | 20.86 lb |
| #6 | 3/4 in | 1.502 lb/ft | 30 in | 30.04 lb |
One-page ordering checklist and worked takeoff table
This checklist turns layout math into an order. It is built for a single-page takeoff: measure, set spacing, apply cover, count linear feet, add splice waste, then convert to sticks. The table below shows common hardscape pours with a buy-now range by bar size.
Ordering checklist
- Confirm the pour shape, including openings and steps.
- Measure overall length, width, and any offsets.
- Choose bar size and spacing.
- Apply edge cover before counting bars.
- Count both directions for slab grids, or straight runs for footings and walls.
- Add lap splice length wherever bars join.
- Add cutoffs for corners, penetrations, and odd pieces.
- Convert total linear feet to 20 ft sticks, then round up.
- Check the weight so hauling and unloading make sense.
| Pour type | Example size | Bar size | Linear feet | Estimated waste | Buy-now range |
|---|---|---|---|---|---|
| Patio slab | 20 ft x 12 ft at 12 in o.c. | #4 | 424 lf | 10% | 24 to 25 sticks |
| Driveway slab | 30 ft x 12 ft at 12 in o.c. | #5 | 648 lf | 10% to 12% | 36 to 39 sticks |
| Footing | 60 lf continuous footing, 2 runs | #4 | 120 lf | 8% to 10% | 7 to 8 sticks |
| Wall | 40 lf retaining wall, vertical and horizontal | #4 | 160 lf | 10% to 15% | 9 to 10 sticks |
For the wall example, the takeoff assumes separate vertical and horizontal bars and does not replace an engineered wall schedule. For the driveway example, wheel loads can justify tighter spacing or a larger bar size. The range is there because field cuts and laps rarely land on a perfect stick count.
Rebar vs wire mesh vs fiber: which one fits the job?
Rebar is the better choice when the concrete needs a real steel grid, longer spans, heavier loads, or continuous support across a footing or wall. Wire mesh often fits smaller slabs and light flatwork where distribution control matters more than structural capacity. Fiber helps with shrinkage control, but it does not replace steel where tension demand is meaningful.
Where rebar makes sense
Use rebar when the slab, footing, or wall needs deliberate reinforcement placement, lap splices, and a defined grid. It also makes sense where edge conditions, openings, or load paths need more control than light reinforcement can provide. That is common in hardscape-adjacent pours and structural elements.
Where mesh is usually enough
Wire mesh can be enough for smaller flatwork when the goal is crack distribution rather than structural capacity. It is lighter to move and often quicker to place. The limit is support and position: if it ends up on the bottom of the slab, it contributes less than intended.
Where fiber helps and where it does not replace steel
Fiber can help control early shrinkage cracking and improve mix cohesion. It does not replace rebar for bending resistance, wall reinforcement, or footing steel. For a small patio or walkway, fiber may reduce the need for a full steel grid, but it should not be treated as a direct substitute in structural work.
Frequently asked questions
how to calculate rebar
Count bars in both directions for a slab grid, or count straight runs for a footing or wall. Subtract for edge cover, divide by spacing, then multiply by run length. After that, add splice waste and convert the total linear feet into 20 ft sticks.
what is the strongest rebar
“Strongest” depends on the job, not only the size number. Larger bars such as #6 and #8 carry more steel area, but they are not always the best choice for every pour. Many residential slabs perform better with correct spacing and cover than with oversized bars placed poorly.
rebar calculator for slab
For a slab, the key inputs are length, width, spacing, cover, and bar size. A good slab calculator counts the grid in both directions, then converts total linear feet into sticks and pounds. It should also flag lap splice waste and openings that interrupt the mat.
rebar calculator for footings
For footings, calculate straight runs instead of a two-way grid. Add extra length at corners, laps, and transitions, then convert the total to 20 ft sticks. If the footing is tied to a wall or step, include those changes in the takeoff so the order covers every run.
rebar calculator weight
Weight is linear feet multiplied by pounds per foot. Use 0.376 lb/ft for #3, 0.668 lb/ft for #4, 1.043 lb/ft for #5, and 1.502 lb/ft for #6. Weight matters because it affects handling, freight, and whether the order will fit the vehicle or trailer.