Concrete Calculator Triangle: Volume & Cost

This concrete calculator triangle tool works out the volume of concrete needed for a triangular slab or footing. You enter the triangle’s base, height, and slab thickness (or depth), and it returns the volume in cubic yards and cubic meters, plus an estimated number of 60-lb or 80-lb bags if you choose that option. The result helps you order ready-mix concrete or bagged mix without over- or under-buying.

Formula

The volume of a triangular prism (which is what a triangular slab is) is:

V = (base × height ÷ 2) × thickness

Where:

• base (b) – the length of one side of the triangle, in feet (or meters).
• height (h) – the perpendicular distance from the base to the opposite vertex, in the same unit as the base.
• thickness (t) – the depth of the concrete, in inches (or centimeters). Convert to the same unit as base and height before multiplying.
• V – volume in cubic feet (or cubic meters).

The first part, base × height ÷ 2, is the area of the triangle. Multiplying that area by the thickness gives the volume. If you measure in feet and inches, convert inches to feet by dividing by 12. For cubic yards, divide the cubic feet by 27 (since 1 cubic yard = 27 cubic feet).

Worked Example

Using the default values from the calculator: base = 10 ft, height = 6 ft, thickness = 4 in.

1. Convert thickness to feet: 4 in ÷ 12 = 0.333 ft.
2. Triangle area = (10 × 6) ÷ 2 = 30 ft².
3. Volume = 30 ft² × 0.333 ft = 10 ft³.
4. Convert to cubic yards: 10 ÷ 27 ≈ 0.37 yd³.
5. 60-lb bags: each bag yields about 0.45 ft³, so 10 ÷ 0.45 ≈ 22.2 → 23 bags.
6. 80-lb bags: each bag yields about 0.60 ft³, so 10 ÷ 0.60 ≈ 16.7 → 17 bags.

The calculator shows 10.00 ft³, 0.37 yd³, 23 bags (60-lb), and 17 bags (80-lb).

When to Use a Triangular Concrete Calculator

Triangular concrete shapes appear in many projects:

• Corner slabs for patios or walkways that meet at an angle.
• Triangular footings for decks or posts at odd angles.
• Stair stringer supports or small ramps.
• Concrete pads for air conditioners or generators that are wedge-shaped.

For any shape that isn’t a rectangle, you need to calculate the area differently. A triangle is half of a rectangle, so the area formula is half the base times the height. The calculator handles that automatically, saving you from a common geometry mistake.

How to Read the Results

The volume result is the exact amount of concrete needed to fill the space. In practice, you should order a little extra – about 5-10% more – to account for spillage, uneven subgrade, or slight measurement errors. For example, if the calculator says 10 ft³, order 10.5 to 11 ft³. For ready-mix, that means ordering 0.4 yd³ instead of 0.37 yd³, since ready-mix is sold in quarter-yard increments.

Bagged concrete yields vary by brand and mix, but a standard 60-lb bag produces about 0.45 ft³, and an 80-lb bag produces about 0.60 ft³. The calculator uses those common values. Always check the bag label for the exact yield, as some high-strength mixes may differ slightly.

Common Mistakes and How to Avoid Them

• Unit mix-ups: Mixing feet and inches without converting. Always convert thickness to feet (or base/height to inches) before multiplying.
• Using the wrong height: The height must be perpendicular to the base, not the length of a slanted side. For a right triangle, the two legs are base and height; for other triangles, measure the vertical distance from the base to the opposite corner.
• Forgetting to divide by 2: The area of a triangle is half the rectangle. Skipping this gives twice the volume.
• Rounding too early: Round only the final volume, not intermediate steps, to avoid significant error.
• Ordering exactly the calculated amount: Add 5-10% for waste and uneven ground.

Limitations of the Formula

This calculator assumes a flat, uniform thickness. If your slab has a slope (like a ramp), the volume is different – you’d need to integrate or use an average thickness. It also assumes the triangle is planar; for curved or irregular shapes, break the area into smaller triangles or use a different method. The bag count is an estimate based on typical yields; actual yields vary by mix and water added. For large projects, ready-mix is usually more economical than bags, but for small patches, bags are convenient.

Alternatives and Related Calculators

If your project isn’t a triangle, you might need a different shape calculator:

• Rectangle slab – length × width × thickness.
• Circle – Ï€ × radius² × thickness.
• Trapezoid – ((base1 + base2) ÷ 2) × height × thickness.
• Post hole – cylinder volume minus the post volume.

Many online concrete calculators handle multiple shapes. For a triangle, you can also use a general volume calculator and manually compute the area first. But a dedicated triangle calculator reduces the chance of arithmetic errors.

Frequently Asked Questions

How do I measure the base and height of a triangular area?

Choose any side as the base. Measure its length. Then measure the perpendicular distance from that base to the opposite corner – that’s the height. For a right triangle, the two shorter sides are base and height. For other triangles, use a square or laser level to find the right angle.

What if my triangle is not a right triangle?

The formula works for any triangle as long as you use the perpendicular height. You don’t need the angles. Just ensure the height is measured at a 90° angle to the base.

How many 80-lb bags do I need for 1 cubic yard?

One cubic yard is 27 cubic feet. An 80-lb bag yields about 0.60 ft³, so 27 ÷ 0.60 = 45 bags. For 60-lb bags (0.45 ft³ each), you need 60 bags. These are standard yields; check your bag for exact numbers.

Can I use this calculator for a triangular footing?

Yes, as long as the footing has a uniform thickness. If the footing tapers (thicker at one end), you need a more complex calculation, like using an average thickness or breaking it into sections.

Should I order extra concrete?

Yes, always order 5-10% more than the calculated volume to cover waste, spillage, and slight over-excavation. For ready-mix, round up to the next quarter or half yard.

What does “yield� mean on a bag of concrete?

Yield is the volume of wet concrete a bag produces after mixing with water. It’s usually listed on the bag, e.g., 0.45 ft³ for a 60-lb bag. Use that number instead of the default if it differs.

Related

• Board & Batten Calculator: Measure & Estimate
• Board and Batten Calculator: Inches & Spacing
• Board and Batten Distance Calculator

in b | Base | ft | 10 | 0
in h | Height | ft | 6 | 0
in t | Thickness | in | 4 | 0
out vol_ft3 | Volume | ft³ | (b * h / 2) * (t / 12) | 2
out vol_yd3 | Volume | yd³ | ((b * h / 2) * (t / 12)) / 27 | 2
out bags60 | 60-lb bags | bags | ceil(((b * h / 2) * (t / 12)) / 0.45) | 0
out bags80 | 80-lb bags | bags | ceil(((b * h / 2) * (t / 12)) / 0.60) | 0