Concrete Calculator

1 cu yd = 27 cu ft · 60 lb bag = ~0.45 cu ft · 80 lb bag = ~0.60 cu ft · Ready-mix sold by cu yd · Short-load fee applies for orders under 3–5 cu yd · Always specify air-entrained mix for exterior slabs in freeze-thaw climates

Always order 10% more than calculated. Confirm current pricing with your local ready-mix supplier before ordering.

Understanding the Calculator Inputs

This calculator converts your project dimensions into cubic yards — the unit ready-mix suppliers use to price and deliver concrete. Getting the inputs right before ordering is critical: running out mid-pour creates a cold joint, a serious structural defect that requires cutting out and re-pouring the affected section.

Length and Width

Measure the actual pour dimensions — the inside of your form boards, not the outside. For irregularly shaped slabs, break the area into rectangles, calculate each separately, and add the cubic yards together. For circular areas (like a round column pad): π × radius² × (thickness ÷ 12) ÷ 27.

Thickness — The Most Important Input

Thickness has a direct 1:1 effect on material volume and cost. A 6-inch slab uses exactly 50% more concrete than a 4-inch slab of the same area. Getting thickness right is the single most important step in concrete estimating. See the thickness guide below for the correct depth by application per ACI 318 and IRC standards.

Why 10% Waste Is Non-Negotiable

Concrete volume estimates assume a perfectly flat, perfectly dimensioned pour. Reality introduces: uneven subgrade (low spots hold more concrete), form boards that flex slightly under concrete weight, and small spillage at the truck chute. Running 5–10% short mid-pour means either stopping to order more — creating a cold joint at the stopping point — or accepting an incompletely filled form. Order the calculated result plus 10%, always.

3 Real-World Concrete Examples

Example 1 — Backyard Patio Slab (20×20 ft, 4 inches, DIY)

A standard residential patio slab. 4-inch thickness, compacted gravel base, rebar on 18-inch grid, DIY pour with rented screed and float.

Real-world note: A 5.5-yard pour is manageable as a DIY project but requires 4–5 helpers minimum — concrete exits the chute faster than most homeowners expect, and you have roughly 60–90 minutes of working time before it starts to stiffen. Rent a bull float and a 10-foot screed board. Have helpers positioned at the far end of the form before the truck arrives. Never add water at the chute — if the mix arrives too stiff, ask the driver to add a small amount of water only, and only before the pour starts.

Example 2 — Two-Car Garage Floor (24×24 ft, 6 inches, Contractor)

Standard 576 sq ft two-car garage slab. 6-inch thickness per code for vehicle traffic, 4,000 PSI air-entrained mix for freeze-thaw climate, rebar on 12-inch grid, vapor barrier under slab.

Real-world note: Garage slabs in freeze-thaw climates must use air-entrained concrete — tiny air bubbles in the mix that give freezing water room to expand without cracking the slab. Standard non-air-entrained mix will develop surface scaling within 3–5 winters if road salt or de-icer is tracked onto the surface. Always specify air entrainment when ordering in Zones 4–7. Ask your ready-mix supplier: “Is this mix air-entrained for exterior exposure?” before accepting the order.

Example 3 — Concrete Footings for a Deck (8 footings, 12-inch diameter × 42-inch deep)

Eight tube form footings for a 16×20 ft elevated deck. 12-inch diameter Sonotube forms, 42-inch depth below frost line in Zone 5, 3,000 PSI mix. Use our concrete footing calculator for exact volume.

Real-world note: At 0.81 cubic yards, bags make more sense than calling a ready-mix truck — minimum order fees alone would exceed the material cost. Mix each footing with 3–4 bags using a portable mixer. Set the post base hardware while the concrete is still wet — position precisely using a string line before the mix stiffens. Re-check alignment 10 minutes after setting.

Concrete Coverage Chart

Quick reference for common slab sizes and thicknesses. All figures are calculated volume — add 10% when ordering.

Calculated volume only. Add 10% to all figures when placing the order.

Thickness Guide by Application

Using the wrong thickness is one of the most expensive concrete mistakes. Here are the correct depths per application per ACI 318 and IRC standards.

Concrete Mix Types & PSI Guide

Concrete is specified by compressive strength in PSI (pounds per square inch at 28-day cure). Never under-specify PSI on structural elements.

Air Entrainment — Critical for Freeze-Thaw Climates

Air-entrained concrete has billions of microscopic air bubbles that provide relief space for water to expand when it freezes. In Climate Zones 4–7 (anywhere with regular freeze-thaw cycles), always specify air-entrained concrete for exterior slabs, driveways, and walkways. The cost premium is typically $5–$10/cu yd. The alternative is surface damage within 3–5 winters.

Water-to-Cement Ratio — Why Not to Add Water

The strength of concrete is determined primarily by the water-to-cement (w/c) ratio. Adding water at the job site can reduce compressive strength by 30–50% — turning 3,000 PSI mix into 2,000 PSI. If the delivered mix seems stiff, request a plasticizer from your supplier — it improves workability without adding water or reducing strength.

Bags vs Ready-Mix: When to Use Each

Ready-Mix Truck Facts

• Truck capacity: 8–10 cu yd typically (confirm with supplier)
• Working time: 60–90 minutes from batching — this is your pour window
• Truck weight: ~66,000 lbs loaded — check driveway and access route weight limits before scheduling delivery
• Chute reach: Typically 15–18 feet from the truck — plan your pour sequence based on chute reach
• Short-load fee: $50–$150 for orders under the supplier’s minimum (usually 3–5 cu yd)

Hidden Costs Most Estimates Miss

1. Subgrade Preparation

Concrete cannot be poured on soft, organic, or poorly compacted soil. Site prep typically includes: excavating 8–12 inches of topsoil, adding 4–6 inches of compacted gravel base, and compacting with a plate compactor. This adds $1–$3/sq ft for a typical slab — $400–$1,200 for a 20×20 ft patio. Use our gravel calculator to estimate the base material separately.

2. Rebar and Reinforcement

Any slab subject to vehicle loads or structural use requires rebar. #4 rebar on 18-inch centers runs $0.60–$1.00/sq ft for the slab area. A 20×20 ft slab needs approximately 280–320 linear feet of rebar. Use our rebar calculator for a precise estimate.

3. Form Lumber

Wood forms (2×4 or 2×6 lumber) are required around the slab perimeter before any pour. For a 20×20 ft slab (80 linear feet of perimeter), form lumber costs $50–$100. Forms can sometimes be reused, but often can’t be removed cleanly after the pour sets.

4. Control Joints

Concrete shrinks as it cures and will crack — control joints give it predetermined places to crack invisibly. Saw-cut control joints should be cut within 6–18 hours of the pour. Saw cutting costs $1–$2 per linear foot. On a 20×20 slab with joints every 10 feet: 40 linear feet = $40–$80 — a service that prevents visible random cracking.

5. Finishing and Sealing

Concrete sealing ($0.25–$0.75/sq ft every 2–5 years) protects the surface from freeze-thaw damage, staining, and wear. Budget $100–$300 for the initial sealing of a residential slab. In climates with road salt, sealing is not optional — it’s the primary protection against surface scaling.

Common Concrete Mistakes

Ordering the Exact Calculated Amount

Running out mid-pour creates a cold joint — the plane where old and new concrete meet. Cold joints are structurally weak and visible on the surface. Running over is free (extra goes in a wheelbarrow). Running short costs a second truck delivery, a cold joint, and potentially the entire slab. Always order calculated amount + 10%.

Adding Water at the Job Site

When concrete arrives stiff, the instinct is to add water. Every gallon added beyond the design mix raises the water-to-cement ratio and reduces strength — potentially by 30–50%. A 3,000 PSI mix with too much water may test at 2,000 PSI. If the mix seems too stiff, the solution is a plasticizer, not water.

Skipping the Gravel Base

Concrete poured directly on topsoil or clay soil settles unevenly as the soil compresses. The result: cracked, heaved, or sunken slabs within 3–5 years. A 4-inch compacted gravel base is required under every residential slab. The gravel base is structural, not optional.

Not Curing Properly

Concrete gains strength through a chemical hydration process that requires moisture. If the surface dries too quickly (hot weather, wind, low humidity), the top layer cures faster than the interior, causing surface cracking. Cure by covering with plastic sheeting for 7 days, applying a liquid curing compound immediately after finishing, or misting the surface 2–3 times per day for a week.

Pouring in Wrong Weather Conditions

Concrete should only be poured when air temperature is between 40°F and 90°F with no rain forecast for 24 hours. Below 40°F, the hydration process slows or stops. Above 90°F, concrete sets too fast, reducing working time and causing shrinkage cracking. Schedule the pour in early morning in hot weather; use heated water in cold weather.

How We Calculate

Volume (cu ft) = Length × Width × (Thickness in inches ÷ 12)

Cubic Yards = Volume ÷ 27 · Order Amount = Cubic Yards × 1.10

This is the industry-standard formula used by contractors, suppliers, and the ACI. Dividing by 27 converts cubic feet to cubic yards (3 ft × 3 ft × 3 ft = 27 cu ft = 1 cu yd). The 10% waste factor accounts for subgrade variation, form flex, and spillage.

One 60 lb bag yields approximately 0.45 cubic feet of mixed concrete. One 80 lb bag yields approximately 0.60 cubic feet. The bag count shown is the 60 lb bag equivalent — shown to help decide whether bags or ready-mix makes sense, not as a recommendation to use bags on larger projects.

Thickness recommendations are per ACI 318 (Building Code Requirements for Structural Concrete) and IRC (International Residential Code). PSI specifications follow ACI recommendations for residential construction. Ready-mix pricing ranges are based on 2026 supplier pricing across 8 US markets, cross-referenced with RSMeans concrete unit cost data.

Frequently Asked Questions