Concrete Calculator - Slabs, Columns, Tubes, Curbs, Stairs & Cost

Concrete Volume Quick Reference

These are pre-calculated cubic yards for a clean 4-inch and 6-inch slab. Real pours rarely match a tidy size, so use this for the truck-side conversation and let the calculator above handle the actual numbers, including waste.

Anything over about 30 bags is the moment to call a ready-mix plant instead of buying sacks.

What "A Yard of Concrete" Actually Means

Nobody at the batch plant says "cubic yard." They say "yard." A yard of concrete is one cubic yard, which is 27 cubic feet, or about 0.765 m³. It weighs around 4,050 lb (1,840 kg), so an extra half-yard is not something you can just shove into a wheelbarrow and forget.

The math is volume in cubic feet (length x width x thickness, all in feet) divided by 27. The single most common mistake is leaving thickness in inches. A 4-inch slab is 0.333 ft thick, not 4. A 6-inch slab is 0.5 ft. Drop the inches into the formula by accident and you order ten times too much concrete - dispatchers take that call about once a month.

Pick The Right Shape First

The calculator switches between five concrete shapes and the SVG diagram redraws to match your numbers. Pick wrong and the volume can be off by 30% or more. A 12-inch round Sonotube 4 ft deep is not the same as a 12 x 12 x 48 inch square - it is pi/4 of that, about 22% less concrete.

Ready-Mix vs Bagged - Where The Cutoff Sits

Rule of thumb in the US: under about half a cubic yard, bags are cheaper and faster. Above one cubic yard, ready-mix wins on cost, finish time, and your back. Between half and one yard, it depends on truck access and how badly you need uniform colour across the pour.

Ready-mix is sold by the cubic yard and delivered in a rotating drum. Minimum delivery is usually 1 to 3 yards. Going under the minimum costs a "short-load fee" of $80 to $200, sometimes more. The truck waits about 5 minutes for free, then charges $2 to $4 per minute. Lose half an hour at the chute and you have spent another $60 to $120. Past about 8 to 10 yards, expect a second truck or a staged pour.

Bagged concrete (QUIKRETE, Sakrete, store brand) comes in 50, 60, 80, and 90 lb bags. An 80 lb bag yields about 0.60 ft³ poured, a 60 lb bag about 0.45 ft³, and a 40 lb bag about 0.30 ft³. To fill one cubic yard at 80 lb you need 45 bags - 3,600 lb of dry mix that has to be lifted twice and mixed before the first batch starts curing. That is the wall where bagged stops working.

PSI / Strength - What To Order

The PSI on the order ticket is the 28-day compressive strength the mix is designed to hit. You match it to the application and, on a permitted job, to whatever the engineer's stamp says. The price difference between 3000 and 4000 PSI is usually only $5 to $10 per yard, so bumping up "just in case" is rarely worth fighting over.

PSI is not the only spec on a mix design. You also see slump (workability), maximum aggregate size, air entrainment (freeze-thaw climates), and admixtures - fly ash, fibres, accelerators, retarders. India uses the M-grade system (M20, M25, M30) instead of PSI; M20 is roughly 2,900 PSI, M25 is about 3,600 PSI, M30 about 4,350 PSI.

Slab Thickness - 4 Inches Is Rarely Just 4 Inches

A 4-inch slab almost never sits on bare dirt. Underneath it lives 4 to 6 inches of compacted gravel or crushed-stone base, a vapor barrier on living-area floors, and possibly rigid foam if the slab is heated. The calculator works with finished concrete thickness, so do not add the base layer to that number.

Common residential thicknesses: 4 inches for interior floors, walkways, and basic patios; 5 to 6 inches for driveways, garage floors, and patios that will carry heavy furniture. Driveways with frequent truck traffic want 6 inches with rebar or fibre. A 4-inch slab carrying a parked F-150 is fine, but it is on the edge - the day a contractor parks a loaded dump truck on it is the day cracks appear.

Edge thickening (a deeper perimeter beam) is standard for monolithic slabs. The slab might be 4 inches in the middle and 12 inches deep at the edge. The calculator does not size edges separately, so for a monolithic pour either average the thickness or estimate the field and the edge beams separately and add them.

Waste Factor - 10% Is Not Optional

Every concrete order needs a waste factor. The 10% default in the calculator matches what most contractors actually order. Real reasons it disappears:

• Forms bow outward under wet weight. A 4-inch slab can become 4.25 inches in the middle if forms are not braced.
• Ground prep is never perfectly level. Low spots eat extra concrete with no warning.
• The truck washout at the end takes some material with it.
• Nobody wants to be the contractor who pays a $200 short-load fee for one extra yard at the end of a pour.

For complicated work (curbs, stairs, multiple lifts, intricate forms) push waste to 15%. For a level interior slab with stiff forms, 5% is enough. Big DOT pours run their own waste factor from job history, sometimes as low as 3%, but they are not figuring it out on the truck.

Cost Estimates Around The World

2026 ready-mix planning prices. The unit changes by region - the US sells by the cubic yard, most of the world sells by the cubic metre. One yd³ equals 0.765 m³, so divide a $/yd³ number by 0.765 to compare with a $/m³ quote.

Extras that show up on the invoice in every market, just under different names:

• Delivery / cartage per truck: roughly $80-200 in the US and Canada, €60-120 in Europe, £50-100 in the UK, ₹1,500-3,500 in India.
• Short-load fee for under-minimum orders: $80-200 (US), £60-120 (UK), €70-140 (EU), ₹1,200-2,500 (India). In Australia, suppliers usually charge per cubic metre with a 0.2 m³ floor; below that you pay the floor.
• Pump truck / boom pump half day: $700-1,500 (US), £400-700 (UK), €500-900 (EU), A$700-1,300 (AU), ₹25,000-45,000 (India), MX$8,000-14,000 (MX), PHP 15,000-25,000 (PH).
• Strength upcharge per 1000 PSI / 5 MPa above standard: about $10 / €10 / £8 / ₹400 per m³.

India note: a cement bag is not a concrete bag. 50 kg of cement plus sand and aggregate at the standard 1:2:4 ratio yields about 0.20 m³ of poured concrete. Brand reference for retail bags: UltraTech, Birla A1, ACC, Ambuja, Dalmia in India; Holcim and CEMEX globally; Boral and Hanson in Australia and the UK. The cement brand matters less than the mix design once it leaves the bag.

Pouring Logistics - The Math Hides The Hard Parts

The volume number is the easy half. What kills pours:

• Truck access. A loaded ready-mix truck needs about 12 ft of overhead clearance, 11 ft of width, and a working turning radius. Back yards usually need a pump. Pump rentals start at $700 for a half day in most US markets.
• Slump. A 4-inch slump means a normal-flow mix. The driver can add water at the chute, but every gallon of extra water cuts strength. If the mix is stiff, ask the plant for a higher slump on the next load instead of adding water on-site.
• Hot weather. Above 90°F (32°C) concrete starts setting fast. Order with a retarder, pour early morning, mist the surface during finishing, and curing blankets stay on longer.
• Cold weather. Below 40°F (4°C) get an accelerator or hot-water mix. Cover with insulated blankets after finishing. Frozen concrete is dead concrete.
• Finish time. A 6-yard residential slab gives you about 90 minutes from chute to broom finish. Plan crew positions before the truck arrives.
• Rain. Light rain in the first hour can pit the surface. Have plastic sheets staged before the pour starts.

Common Mistakes

• Using slab math for round columns, tubes, curbs, or stairs.
• Entering thickness in inches when the field expects feet, or vice versa.
• Leaving the quantity at 1 when there are multiple identical pads, columns, or stair flights.
• Ordering exact yards with no waste allowance and running out at the back of the pour.
• Confusing 60 lb and 80 lb bag yields - they are not the same.
• Treating a cement bag count as a concrete bag count. Cement is one ingredient in concrete, not the whole thing.
• Skipping the short-load fee in the budget for a small pour.
• Pouring on uncompacted soil or skipping the gravel base; the slab cracks within a few seasons.

Related Construction Calculators

Pour-specific tools: Slab Calculator for plain rectangular pours, Footing Calculator for strip, pad, and pier footings, and Cement Calculator if you are starting from bagged cement and ingredients. For reinforcement, see the Rebar Calculator. Stair layout sits in the Stair Calculator. Footprint cleanup usually starts with the Square Footage Calculator. More tools sit on the Construction Calculators hub.