Why bar count is floor(effective ÷ spacing) + 1
Why the +1 is not optional
Bars sit on-center. If you have 18 feet of effective width and 18-inch spacing, you do not have 18 ÷ 1.5 = 12 spaces and 12 bars. You have 12 spaces and 13 bars — one at each end plus the ones between. The formula is . Dropping the +1 is how a 20-foot mat ships one bar short on every row.
Effective width is the slab or footing dimension minus cover on both edges. Cover is not decorative. ACI 318 and IRC tables set it so the bar has concrete around it. Weight is linear feet times the ASTM pounds-per-foot for that size. The ASTM table on the right already lists those unit weights; this guide does not reprint it.
The 40d lap shown here is a Grade 60 tension-lap approximation (40 × bar diameter). It is not the lap on your drawing. Class A/B, epoxy, lightweight concrete, and required development length change that number. Do not cut bars to 40d because this page printed it. Compression laps are often shorter; hook bars are a different takeoff entirely.
Worked field example
22 × 18 foot mat, #5 at 18 inches on center, 3-inch edge clearance, two-way. Not the 20 × 20 / #4 / 12-inch default.
Effective length is 22 − 2 × 0.25 = 21.5 feet. Effective width is 18 − 0.5 = 17.5 feet. Spacing is 1.5 feet. Bars along the 22-foot side: floor(17.5 ÷ 1.5) + 1 = 12. Bars along the 18-foot side: floor(21.5 ÷ 1.5) + 1 = 15. Without the +1 you would have ordered 11 and 14 and left a gap at the far edge.
#5 weighs 1.043 lb/ft. Net steel is 12 × 21.5 + 15 × 17.5 = 520.5 feet, about 543 pounds before waste. A 40d lap on #5 is 25 inches — use it only as a reminder to look at the schedule, not as a cut list.
Cover and grade — what this page does not design
ASTM A615/A706 fix the bar you can buy. ACI 318 and IRC R506/R403 set cover and when a slab needs steel at all. This engine counts bars. It does not check temperature steel, shrinkage spacing, or whether welded-wire is allowed in place of the mat.
Cover and product — not a bar-size design
- Provision
- ACI 318 cover
- Limit
- Often 3 in to earth, 1.5 in to forms, 3/4 in in slabs (check the adopted edition)
- Failure mode
- Using 3-inch clearance in a formed wall that only needs 1.5
- Provision
- ASTM A615 / A706
- Limit
- Deformed bar grade and weldability
- Failure mode
- Ordering A615 where the spec wants A706 for welding
- Provision
- 40d sketch
- Limit
- Grade 60 tension lap approximation only
- Failure mode
- Cutting laps from this page instead of the structural sheet
| Provision | Limit | Failure mode |
|---|---|---|
| ACI 318 cover | Often 3 in to earth, 1.5 in to forms, 3/4 in in slabs (check the adopted edition) | Using 3-inch clearance in a formed wall that only needs 1.5 |
| ASTM A615 / A706 | Deformed bar grade and weldability | Ordering A615 where the spec wants A706 for welding |
| 40d sketch | Grade 60 tension lap approximation only | Cutting laps from this page instead of the structural sheet |
Where the field usually goes wrong
Omitting the +1 is the counting error. The other is treating welded-wire mesh as the same takeoff. WWM is ordered in rolls or sheets by square feet, not as #4 at 12 inches. Converting a W6×W6 sheet into “equivalent bars” and then running this calculator will not match the bundle the supplier sends.
Laps consume extra length that a simple grid count does not see unless you add them or raise the waste slider. A 20-foot stock bar in a 22-foot pour needs a splice; 40d is the reminder, not the cut. Mill lengths are 20, 40, and 60 feet — a 21.5-foot effective run is two 20s plus a lap, not one mythical 21.5-foot stick. Chairs and dobies are not in the pound total. Two-way versus one-way matters: uncheck the mat and you only get bars in one direction.
Frequently asked questions
Why add one after dividing by spacing?
On-center spacing counts gaps. floor(effective ÷ spacing) is the gaps; +1 is the bars at both ends.
Is 40d the lap on my drawing?
No. It is a Grade 60 sketch (40 × diameter). Class A/B, epoxy, and fy change it. Cut from the schedule.
Can I use this for welded-wire mesh?
No. Mesh is a sheet product. Order WWM by coverage, not as #4 equivalents.
Does edge clearance equal ACI cover?
It is the setback this page uses so the first bar is not on the form. Legal cover is the adopted table. Match the drawing.
One-way or two-way — which do I pick?
A slab mat is two-way. Bars only along a footing: uncheck the mat.
Where do the lb/ft numbers come from?
ASTM A615 nominal weights. The table to the right lists them. This guide does not copy it.
Does this replace the structural drawing?
No. Size, spacing, cover, and lap are design. This is a takeoff of a grid you already chose.