AtlasCalcsUtility Calculation Suite
2026-09-23 8 min read

Concrete Slab Calculator

Free concrete slab calculator — calculate slab concrete volume with waste, bag counts, truckloads and cost. Includes live results, step-by-step working, charts, material takeoff and cost estimates. No sign-up.

Calculator

Length(ft)
ft
Width(ft)
ft
Thickness(in) Help
in

Slab thickness

Waste allowance(%) Help
%

Bulge and spillage

Price (optional)($/cu yd) Help
$/cu yd

Ready-mix or bag rate

Bag size

Pour plan

ItemAmount
Net volumeCalculated2.963 cu yd
Order volumeNet + waste — order this3.170 cu yd
BagsRounded up143 bags
TrucksAt 10 cu yd each1

How to use this result

  • Primary result: 3.17 cu yd — verify it against a rough hand check before ordering.
  • Read the steps section to see every conversion and the exact formula path.
  • Use the breakdown table on the left as your order or bid checklist.
  • Re-run with worst-case dimensions so the plan covers the whole job.

Results

Optimal

Order volume

3.170cu yd

Net plus waste — order this

Bags needed

143bags

Rounded up, selected size

Ready-mix trucks

1loads

At 10 cu yd per truck

Net cubic feet

80.0cu ft

Before waste

Live results update as you type — currently showing 3.17 cu yd. Replace the defaults with your own measurements for exact results.

Visualizations

Order vs truck capacity (cu yd)

3.2Your order
10Full truck

Net volume vs waste

Net93%
Waste7%

Formula

Slab volume is length times width times thickness in feet, divided by 27 for cubic yards.

V = L × W × (T ÷ 12) ÷ 27

Variables

SymbolNameUnitDescription
VVolumecu ydOrder unit for ready-mix.
L, WLength, widthftPlan dimensions.
TThicknessinConverted to feet (÷ 12).

Explanation

Thickness converts to feet, cubic feet divide by 27, waste multiplies, then bags (÷ yield, round up) and trucks (÷ 10, round up) divide from the order.

Step-by-Step Calculation

Follow each step to understand how the result is calculated from your inputs.

Step 1

Convert thickness to feet and find cubic feet.

Length:20 ft
Width:12 ft
Thickness:4 in → 0.33 ft
Result: 80.00 cu ft
Step 2

Divide by 27 for cubic yards.

V = 80.00 ÷ 27
Result: V = 2.963 cu yd
Step 3

Add 7% waste.

Order = 2.963 × 1.07
Result: Order = 3.170 cu yd
Final Answer
3.170

Worked Examples

See how the calculator works with different scenarios.

Scenario

The calculation run on the values above, showing the full path rather than only the answer.

Inputs

Result:3.17 cu yd

Calculation

1. Convert thickness to feet and find cubic feet. → 80.00 cu ft 2. Divide by 27 for cubic yards. → V = 2.963 cu yd 3. Add 7% waste. → Order = 3.170 cu yd

Result

3.17 cu yd

Interpretation

Round up to whole bags and half trucks, keep 5–10% waste in the order, and confirm subgrade depth with probes before scheduling.

Understanding Your Result

Your Result: Order volume: 3.170 cu yd

Typical range: Sidewalk square ~9 cu ft · patio 12×12×4 in ~1.8 cu yd · driveway 20×12×4 in ~3 cu yd · truck ~10 cu yd.

What This Means

Your result is 3.17 cu yd — the order volume, bag count, truckloads and cost for your slab

Why It Matters

Concrete is unforgiving: short loads stop pours, long loads waste money, and weak mixes fail inspections. Modelling volume, bags and cost first is the cheapest insurance on the job.

Professional Interpretation

Round up to whole bags and half trucks, keep 5–10% waste in the order, and confirm subgrade depth with probes before scheduling.

How this is calculated

Volume is length × width × thickness-in-feet, divided by 27 for cubic yards. Waste multiplies last; bags round up; trucks round up to whole loads.

What drives the result

The inputs in a single consistent unit and the allowance or price you apply.

Recommendations

  • •Verify critical dimensions on site before ordering.
  • •Keep one unit throughout each calculation.
  • •Add the trade allowance before buying, not after.
  • •Attach the step-by-step working to bids and records.

Educational Content

Introduction

Concrete quantity is volume: length × width × thickness, converted to cubic yards for ordering. Bags, trucks and cost all divide from the same order volume.

Concept Explanation

Volume is length × width × thickness-in-feet, divided by 27 for cubic yards. Waste multiplies last; bags round up; trucks round up to whole loads.

Applications

▸Slab, footing, wall and column volume takeoffs
▸Bag counts for site-mixed pours
▸Ready-mix truck scheduling and short-load checks
▸Pour cost estimates with labor and delivery
▸Mix ratio batching for cement, sand and gravel

Benefits

  • •One live number for volume, bags, trucks and cost
  • •Waste allowance built into every order row
  • •Steps show the exact formula path for records
  • •Free, no sign-up, works on site from a phone

Limitations

  • •Two-dimensional math unless stated otherwise — slopes and curves need the right tool
  • •Results are only as good as the site measurements
  • •Allowances and prices are estimates — confirm with local quotes

Best Practices

  • •Measure twice and write measurements down before typing them in
  • •Keep one unit throughout each calculation
  • •Photograph the tape on large or disputed measurements
  • •Re-run with worst-case numbers so the plan covers the whole job

Common Misconceptions

Myth: The calculator replaces site verification.
Truth: It replaces the arithmetic, not the tape measure — always verify before ordering.
Myth: One decimal place is always enough.
Truth: On long runs, rounding early compounds. Enter full measurements and let the tool round at the end.
Myth: Metric and imperial give different answers.
Truth: They are the same physical quantity in different units — conversion is exact.

Practical Tips

  • •Enter full measurements and let the tool round at the end.
  • •Use the steps section to double-check the formula path.
  • •Save or share the result link with the crew and the supplier.
  • •Re-run with worst-case dimensions for a safe order.

Common Mistakes

Rounding measurements before calculating.
Enter the full tape reading; rounding early compounds across a job.
Mixing units between fields.
Pick one unit in the dropdown so every conversion is consistent.
Forgetting to verify on site.
Plans differ from as-built work — confirm critical dimensions.
Ordering the bare result with no allowance.
Add the trade waste or contingency factor before buying.

Assumptions

This calculator makes the following assumptions to provide estimates:

Consistent units: All inputs share the selected unit system.
Standard geometry: Flat-plane Euclidean formulas unless stated.
Full precision: Rounding happens only on display, never mid-calculation.

Limitations

Be aware of these limitations when using this calculator:

Estimates, not quotes: Allowances and unit prices vary by region — get 2–3 local quotes.
Verify on site: Confirm critical dimensions before ordering or cutting.

Frequently Asked Questions

Answers to common questions about this calculator.

How thick should a slab be?
4 inches for walks and patios, 4–6 for driveways (6 at the apron), 4–6 for slabs with the right base and joints. Thickness drives volume directly — verify with probes.
How do I calculate concrete for a slab?
Multiply length × width × (thickness in feet), divide by 27 for cubic yards, then add 5–10% waste.
How many bags of concrete do I need?
Divide the order volume by the bag yield: 0.60 cu ft per 80 lb bag, 0.45 per 60 lb bag. The bags row rounds up to whole bags.
How many cubic yards in a concrete truck?
About 10 cubic yards (270 cu ft). Short loads under ~4–5 yards carry a fee — the delivery rows flag it.
Should I add waste to concrete orders?
Yes — 5–10% for form bulge, spillage and uneven subgrade. Running short mid-pour creates a cold joint.
How accurate is this calculator?
Exact for the dimensions entered. Probe the subgrade and check forms — depth variation is the usual error.
What does the cost estimate include?
Material at your unit price on the order quantity. Labor, pump, reinforcement and delivery fees add per the cost rows.
When should I use ready-mix instead of bags?
Above roughly 1 cubic yard (about 45 eighty-pound bags), ready-mix is cheaper and far faster. Below that, bags win.
How many 80 lb bags are in a cubic yard?
45 bags (27 divided by 0.60). Past about a yard, ready-mix is usually cheaper and far faster than bags.
How thick should my slab be?
4 inches for walks and patios, 4–6 for driveways (6 at the apron), 4–8 for slabs carrying loads — with the right base, joints and reinforcement per the drawings.
How much does ready-mix cost per yard?
Roughly $125–$200 per cubic yard plus delivery, short-load and pump fees depending on region and mix. Enter your plant quote for a live total.
What is a short-load fee?
Plants charge extra below about 4–5 yards because the truck runs half empty — sometimes more than the concrete itself. Combine small pours.
How do I prepare the subgrade before a pour?
Compact undisturbed or engineered fill, hold grade with probes, lay 4 in of gravel where specified, and wet (not muddy) dry subgrade so it does not steal mix water.
How far apart do control joints go?
About 24–30 times the slab thickness in inches (8–10 ft for a 4 in slab), cut one-quarter deep within a day. Joints plan the cracks.
When can I drive on new concrete?
Light cars after about 7 days of good curing; heavy trucks after 28. Early loading cracks slabs permanently.
What causes concrete cracks?
Shrinkage, missing or late joints, rapid drying, weak subgrade, overload and freeze damage. Cure 7 days and joint properly to prevent most of it.
Should I order in the morning or afternoon?
Mornings in heat (more working time, same-day finish), midday in cold (warmest placement window). Wind and low humidity demand faster curing protection.
Pump, chute or buggy — how do I place it?
Chutes reach about 12–15 ft from the truck; pumps reach anywhere but cost; buggies bridge short gaps. Price access before the pour, not during.
How do I estimate labor for placing and finishing?
Crew size times hours at local rates — flatwork crews place 8–15 yards/hour by chute. The pour planner totals crew cost from your rate.
What strength concrete should I order?
3,000 psi for walks and patios, 3,500–4,000 for driveways and slabs with loads, air-entrained where freeze-thaw hits. Strength is specified, not guessed.
Does the concrete slab calculator work on a phone at the job site?
Yes — every page is mobile-friendly with no sign-up. Bookmark the tools you use daily for one-tap takeoffs.
How often should I re-run the concrete slab calculator?
Re-run whenever dimensions change, prices move, or you switch materials. Early estimates, pre-order checks and final buy quantities each deserve a fresh run.
What should I check before ordering materials?
Dimensions verified on site, correct unit, waste allowance added, pack sizes and minimums confirmed, delivery access and schedule agreed. The breakdown table is your checklist.
Where do I find the formula behind the concrete slab calculator?
In the formula section below the results, with every variable defined, plus a worked example tracing your own inputs step by step.
How do I read the steps section on the concrete slab calculator page?
Each numbered step shows the inputs used, the formula applied and the intermediate result, ending in the headline figure. Follow it top to bottom to audit the math.
What if my site conditions differ from the page defaults?
Defaults are worked examples, not recommendations — replace every input with your own measurements, prices and allowances before ordering or bidding.
How do I budget for price changes mid-project?
Lock supplier quotes in writing with expiry dates, carry a 5–10% escalation contingency, and re-run the calculator when prices move. Volatile materials get shorter quote windows.
Should I buy materials in bulk for this job?
Bulk wins when storage is dry and secure, the material keeps, and the discount beats carrying cost. Otherwise phased delivery avoids damage, theft and cash lockup.
How do I document quantities for change orders?
Record before-and-after measurements with dated photos, keep delivery tickets, and re-run the calculator for the changed scope. Paper trails get paid; memories do not.
What inspections relate to the concrete slab calculator?
Anything structural, concealed or regulated gets inspected before covering: footings, steel, drains, waterproofing and rough-ins. Book inspections before the work that buries them.

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Related Guides

Deepen your understanding with these educational resources.

Glossary

Technical terms used in mortgage calculations explained in plain language.

Cubic yard (cu yd)
27 cu ft — the unit ready-mix is sold in. A full truck carries about 10 cu yd.
Bag yield
Finished concrete per bag: 0.60 cu ft (80 lb), 0.45 (60 lb), 0.375 (50 lb), 0.30 (40 lb).
Waste factor
Extra 5–10% for form bulge, spillage and uneven subgrade.
Slump
Inches a fresh cone settles — the field measure of workability.
Water-cement ratio
Water weight ÷ cement weight; lower is stronger (≈0.40–0.60).
Cure
Keeping concrete moist and warm so hydration completes — 7 days typical.
cu yd
The primary unit reported by this calculator.

Authoritative References

Trusted sources and standards referenced by this calculator.

Applicable Standards

ACI 318: minimum cover, strength and durability for structural concreteOrder 5–10% extra for bulge, spillage and grade variation1 cu yd = 27 cu ft; full ready-mix truck ≈ 10 cu ydVerify subgrade compaction and form dimensions before the pour

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Page Information

Last Updated:

2026-09-23

Version:

2.0.0

License:

Free to use for educational and professional purposes.