Concrete Bag Calculator

Calculate the volume of your project in cubic feet, convert to cubic yards (divide by 27), then divide by the yield per bag. An 80 lb bag yields 0.6 cu ft, a 60 lb bag yields 0.45 cu ft, and a 40 lb bag yields 0.3 cu ft. For example, a 4 × 4 ft slab at 4 inches thick = 5.33 cu ft ÷ 0.6 = 9 bags of 80 lb concrete. Always add 10% for waste.

80 lb bags are the best value and most popular for general projects — one bag fills about 0.6 cu ft. 60 lb bags are a good middle ground if 80 lbs is too heavy to lift. 40 lb bags are easiest to handle and work well for small repairs and fence posts. For large projects (over 1 cubic yard), consider ordering ready-mix concrete delivered by truck instead — it’s cheaper and faster.

A standard 4×4 fence post in a 10-inch diameter hole at 24 inches deep needs about 1 bag (80 lb) of concrete. A 6×6 post in a 12-inch hole at 30 inches deep needs 2 bags. Formula: hole volume = π × radius² × depth − post volume. Set the post, fill with dry concrete mix, then add water — or pre-mix in a wheelbarrow for a stronger result.

Mix in a wheelbarrow or mixing tub for small batches (1–3 bags). Use a mixing drill with a paddle attachment for medium jobs. Add water gradually — the mix should hold its shape when squeezed but not be crumbly. Too much water weakens concrete. Mix for 3–5 minutes until uniform color with no dry pockets. Work in batches you can place within 15–20 minutes.

Concrete sets (hardens enough to hold shape) in 24–48 hours. It reaches working strength in 3–5 days. Full cure takes 28 days to reach maximum strength. Fast-setting concrete (like Quikrete Fast-Setting) sets in 20–40 minutes and reaches working strength in 1–2 hours. Temperature affects curing: below 50°F slows it significantly, above 90°F requires extra water and shade.

An 80 lb bag costs $5–$8 and yields 0.6 cu ft, making the cost per cubic yard about $225–$360. Ready-mix delivered by truck costs $125–$175 per cubic yard with typical minimum orders of 1–3 yards. Bag concrete is cheaper for projects under 0.5 cubic yards. Above that, ready-mix saves money and significant labor. The break-even point is roughly 45–60 bags (0.5–0.65 cu yd).

The standard water ratio is about 3 quarts (0.75 gallons) per 80 lb bag, 2.25 quarts per 60 lb bag, and 1.5 quarts per 40 lb bag. Start with slightly less water and add gradually — you can always add more, but you can’t take it out. Concrete that’s too wet will be weak and prone to cracking. The ideal consistency resembles thick oatmeal or peanut butter.

Light foot traffic is safe after 24–48 hours for standard concrete or 2–4 hours for fast-setting mixes. Wait 3–5 days before placing heavy objects or building on the slab. Drive vehicles on concrete after 7–10 days. Full load capacity is reached at 28 days. Keep concrete moist during the first 7 days of curing — cover with plastic or mist regularly for strongest results.