How to use
- Measure the slab rectangle and enter its specified concrete thickness.
- Set an allowance for the concrete pour; it does not change the joint grid.
- Enter the compacted stone subbase depth, or use zero if it is outside your scope.
- Read the concrete volume, bag equivalent, and preliminary interior joint counts together.
How the estimate is calculated
Net concrete is length × width × thickness ÷ 12 ÷ 27 in cubic yards. The order quantity multiplies this by 1 + extra percentage ÷ 100. The suggested maximum joint spacing is the smaller of 30 × thickness ÷ 12 and 15 feet. Each direction gets ceiling(length ÷ spacing) − 1 interior joint lines, with a minimum of zero.
Example: for a 10 ft × 10 ft slab at 4 in, with 10% extra and a 4 in stone subbase, the net volume is 1.23 cubic yards and the order is 1.36 cubic yards, equivalent to 62 bags of 80 lb mix. Spacing is 10.00 ft, so this rectangle needs 0 interior lines in either direction under this simplified grid. Joint groove depth is 1.00 in; estimated subbase weight is 2.28 short tons.
Joint-planning numbers from NRMCA CIP 6
| Item | Guidance or calculation | How it is used here |
|---|---|---|
| Spacing range | 24–36 times slab thickness | Calculator selects a factor of 30 |
| Spacing cap | 15 ft | Limits the calculated spacing |
| Groove depth | At least one-quarter thickness and at least 1 in | Larger of those two values |
| Panel proportions | Length no more than 1.5 times width | Review panel geometry separately |
Source: NRMCA CIP 6, Joints in Concrete Slabs on Grade. A calculated grid can still produce narrow panels, so review panel proportions and openings before laying out cuts. Quikrete 1101 gives a different product installation example of joints every 6–8 ft; follow the applicable project and product instructions.
Thickness, edges, and the stone layer
Enter a designed thickness rather than choosing one solely from a material calculator. Loading, soil support, reinforcement, and exposure are outside this estimate. The stone calculation uses area × compacted depth and a commonly used planning figure of 1.85 short tons per cubic yard. Ask your aggregate supplier to confirm the weight for the specified material.
This page covers a uniform slab only. Thickened edges, grade beams, openings, and isolated pads require separate volume takeoffs; the joint output does not account for them. Use the general concrete calculator to estimate additional rectangular or round pours.
Common mistakes
- Do not mistake a count of interior joint lines for the total number of panels.
- Do not add stone depth to concrete thickness; they are separate materials.
- A saw-cut depth result does not decide when to cut or how to handle reinforcement at a joint. Consult the jointing plan.
Sources
Joint spacing, panel geometry, and groove depth: NRMCA CIP 6. Bag equivalent and alternate joint example: Quikrete 1101. The 1.85 stone factor is a commonly used planning figure; confirm it with the aggregate supplier.
Related calculators
- Concrete Calculator — separate columns and additional pours
- Gravel Calculator — compare aggregate quantities
Frequently asked questions
Does this include thickened slab edges?
No. The volume assumes a uniform rectangle. Measure a thickened edge separately, counting only the additional depth below the slab to avoid overlap.
Why does a 4-inch slab show 10-foot spacing?
The planner chooses 30 times thickness: 30 × 4 inches = 120 inches, or 10 feet. This is a preliminary selection within the NRMCA range.
Why is joint depth sometimes more than one-quarter thickness?
The calculation also applies the 1-inch minimum described in NRMCA CIP 6, so a thin slab can show a groove deeper than one-quarter of its thickness.
Does changing the allowance change the stone weight?
No. The concrete allowance affects concrete volume and bags. Stone weight is based on the entered compacted subbase dimensions without that extra percentage.