Show the math
- Convert to feet (inches ÷ 12, centimeters ÷ 30.48): Curb depth 6 in = 0.5 ft; Curb height 6 in = 0.5 ft; Gutter width 18 in = 1.5 ft; Flag thickness 6 in = 0.5 ft; Length 100 ft
- Curb section = 0.5 × 0.5 = 0.25 ft²
- Flag section = (0.5 + 1.5) × 0.5 = 1 ft²
- Volume = (0.25 + 1) × 100 × 1 = 125 ft³
- Net volume = 125 ft³ ÷ 27 = 4.6296 yd³ (× 0.0283168 = 3.5396 m³)
- Order quantity = 4.6296 yd³ × (1 + 10%) = 5.0926 yd³ = 137.5 ft³
- 80 lb bags = 137.5 ft³ ÷ 0.6 ft³ per bag = 229.17 → 230 bags (always round up)
- Weight = 137.5 ft³ × 150 lb/ft³ = 20,625 lb
How to measure
Curb depth is the horizontal thickness of the upright curb. Curb height is its vertical height. The gutter width is the flat flag in front of the curb, and the flag thickness is how thick that flag is. The calculator treats the flag as running under the curb, which is how curb and gutter is normally cast.
Length is the run along the centerline of the curb. On a curve, measure along the middle of the curb, not the inside edge.
The curb and gutter formula
The cross-section is the curb rectangle plus the flag rectangle. The curb rectangle is depth × height. The flag rectangle is (depth + gutter width) × flag thickness, because the flag runs the full width under the curb as well as in front of it. Multiply the section area in square feet by the run length in feet to get cubic feet.
All the section dimensions are in inches. Convert them to feet by dividing by 12 before multiplying. A 6 in curb 6 in high has a section of 0.25 ft², and the flag adds 1.0 ft², so the section is 1.25 ft² for the example below.
Formula: V = (D × H + (D + G) × F) × L × quantity, all in feet.
Worked example: 100 ft of curb and gutter
- Convert to feet (inches ÷ 12, centimeters ÷ 30.48): Curb depth 6 in = 0.5 ft; Curb height 6 in = 0.5 ft; Gutter width 18 in = 1.5 ft; Flag thickness 6 in = 0.5 ft; Length 100 ft
- Curb section = 0.5 × 0.5 = 0.25 ft²
- Flag section = (0.5 + 1.5) × 0.5 = 1 ft²
- Volume = (0.25 + 1) × 100 × 1 = 125 ft³
- Net volume = 125 ft³ ÷ 27 = 4.6296 yd³ (× 0.0283168 = 3.5396 m³)
- Order quantity = 4.6296 yd³ × (1 + 10%) = 5.0926 yd³ = 137.5 ft³
- 80 lb bags = 137.5 ft³ ÷ 0.6 ft³ per bag = 229.17 → 230 bags (always round up)
- Weight = 137.5 ft³ × 150 lb/ft³ = 20,625 lb
| Quantity | Result |
|---|---|
| Net volume | 4.630 yd³ (125.00 ft³, 3.540 m³) |
| Order with 10% waste | 5.09 yd³ (3.89 m³) |
| 80 lb bags | 230 |
| 60 lb bags | 306 |
| Weight at 150 lb/ft³ | 20,625 lb |
That is 5.09 yd³ for 100 ft with waste. Without waste it is 1.25 ft³ per linear foot, so you can multiply that figure by any length to check a quote.
Curb sections vary
There is no single curb. Barrier curbs, mountable curbs and rolled curbs have different shapes, and agencies publish their own standard drawings. This calculator uses rectangles, which is accurate for a plain upright curb. A sloped or rolled face removes some concrete, so the result is slightly high and safe to order from.
For public work, use the standard detail from the agency. For a driveway apron or a garden edge, the rectangular model is more than good enough.
Practical tips
- Curb machines and hand forms both need a stiff mix. Ask for a low-slump mix and expect a plasticizer if the plant recommends it.
- Plan control joints through the curb at the same spacing as the flag.
- Pour curb and gutter monolithically so there is no cold joint where water runs.
- For a short run, such as a parking-lot island, bagged concrete is realistic: 10 ft is only 12.5 ft³, about 21 bags.
Frequently asked questions
How do I calculate concrete for a curb and gutter?
Find the cross-section area: curb depth × curb height plus (curb depth + gutter width) × flag thickness, in square feet. Multiply by the length in feet. Divide by 27 for cubic yards.
How much concrete is in 100 ft of curb and gutter?
With a 6 × 6 in curb, 18 in gutter and 6 in flag, about 125 ft³ or 4.63 yd³ before waste.
What is a flag in curb and gutter?
The flag is the flat gutter pan in front of the curb face that carries water along the edge of the road.
Does this work for rolled or mountable curb?
It treats the section as rectangles, so a sloped face is slightly overstated. That is a safe direction for ordering.
How much waste should I allow?
Five to ten percent. Hand-formed runs lose more than machine-placed curb.
Can I use this for a concrete mow strip or edging?
Yes. Set the gutter width and flag thickness to the size of the strip, or set the curb height to zero if your design is flat.
Sources
Estimate only. Results depend on the numbers you enter, ground conditions and your supplier. For structural work consult a licensed engineer and follow local codes. Brand names such as Quikrete and Sakrete are used only to cite published product data. This site is not affiliated with any brand.