Concrete Calculator for Slabs, Footings, Columns and Steps

How to use the concrete calculator

1. Add a row for each shape. A typical backyard project might be a patio slab, a perimeter footing, a few round column forms for a pergola and a set of entry steps. Use the buttons under the list to add each one.
2. Fill in the dimensions. Lengths and widths are in feet; thickness, diameters of round forms and step rise/run are in inches — the units contractors actually use for each one.
3. Use quantity for repeats. Four identical piers? One row with a quantity of 4.
4. Set a waste allowance. 5–10% covers spillage, over-excavation and form bulge.

Each row shows its own volume so you can see which part of the job drives the order. The total converts to cubic yards (or cubic meters), the nearest quarter-yard ready-mix order, and 80 lb and 60 lb bag counts. For how many bags of each size make a cubic yard, see the concrete bag calculator.

Concrete volume formulas by shape

Steps

Steps poured solid on grade are a stack of blocks. Step 1 (the bottom) is one rise tall, step 2 is two rises tall, and so on; every step is one tread deep, except the top step, which is as deep as the landing:

For 3 steps that are 4 ft wide with a 7 in rise, 11 in treads and a 36 in landing: (7×1×11 + 7×2×11 + 7×3×36) × 4 ÷ 144 = 27.4 cu ft, or just over a cubic yard. Steps use far more concrete than people expect, almost all of it in the landing.

Worked example: patio, pergola piers and steps

• Patio slab 12 × 16 ft at 4 in: 12 × 16 × 4 ÷ 12 = 64.0 cu ft
• 4 round columns, 12 in diameter × 4 ft deep: 4 × π × 0.5² × 4 = 12.6 cu ft
• 3 steps as above: 27.4 cu ft

Total: 104.0 cu ft. Add 10% and you need 114.4 cu ft, or 4.24 cubic yards. A ready-mix plant would take an order of 4.25 yards; in bags it’s 191 × 80 lb — a strong case for the truck.

If the truck can’t reach the forms, ask about a pump or a power buggy. Moving 114 cu ft by wheelbarrow, at 2–3 cu ft per load, is roughly 40–60 trips.

Ordering ready-mix: waste factor and tips

• Round up, never down. Plants batch in quarter- or half-yard increments. Being short a quarter yard means a second delivery fee and possibly a cold joint.
• Ask about short-load fees. Many plants have a minimum order (often several yards) and charge a fee below it. Combining jobs — the footing and the slab on the same day — can avoid it.
• Specify strength and slump. Typical residential flatwork is 3,000–4,000 psi; exterior slabs in freeze-thaw climates usually need air-entrained mix. Your local code and supplier can advise.
• Check your subgrade depth. If the base is 1 in lower than planned across a 12 × 16 slab, that’s 16 extra cubic feet — 25% more concrete than the 64 cu ft you planned for a 4 in slab. Use a string line before you order.
• Have a plan for leftovers. Set up a small form for a splash block or stepping stones so extra concrete has somewhere to go.

Just setting a few posts or a small pad? The concrete bag calculator is quicker and subtracts the post for you. For the gravel base under the slab, use the gravel calculator.

Quick reference

Sources & assumptions

How this calculator gets its numbers. Full method: our methodology.

• Volumes use standard geometry for slabs, footings, round columns and steps; 27 cu ft = 1 cubic yard.
• Bag counts use the manufacturer's published yields (80 lb ≈ 0.60 cu ft).
• The default 10% extra is an ordering allowance, not a structural figure. Bags are rounded up to whole bags.

Sources

1. QUIKRETE, Concrete Mix No. 1101 product data sheet. Bag yields: 40 lb ≈ 0.30 cu ft, 50 lb ≈ 0.375, 60 lb ≈ 0.45, 80 lb ≈ 0.60 cu ft.
2. ASTM International, ASTM C387/C387M, Standard Specification for Packaged, Dry, Combined Materials for Concrete and High Strength Mortar. The product standard bagged concrete mixes are made to.

Last reviewed October 25, 2026. Product figures vary by brand; the label on what you buy wins.