Concrete Calculator
Use this concrete calculator before you order a single bag. Pick your shape, type the size, and read the full order in one result. The tool shows cubic yards to order, bag counts for all four bag sizes, the weight in tons, and the cost of the bag route and the ready mix route side by side. It suits a patio slab, a driveway, deck footings, fence post holes and a shed base.
| Bags of 40 lb | 123 bags |
| Bags of 50 lb | 98 bags |
| Bags of 60 lb | 82 bags |
| Bags of 80 lb | 62 bags |
What the result numbers mean
Your pour volume with the extra percent added. Order this amount and the pour should not run short.
The bare math of the shape you measured. The gap between this number and the order is your safety margin.
Every common bag size counted at once. Each count includes your extra percent and rounds up to whole bags.
Yards turned into tons, at about 2 tons per cubic yard. Use the weight to judge truck trips and trailer loads.
Type a price per bag or a price per cubic yard. The tool prices both routes on your real job size, so the cheaper route shows itself.
The same result in metric units. One set of measurements serves both unit worlds.
How much concrete do I need?
Measure the length, the width and the thickness. Multiply them to get cubic feet, then divide by 27 for cubic yards. A 10 by 10 foot slab at 4 inches thick needs 1.23 cubic yards before extra, and 62 bags of 80 lb with a 10 percent allowance. The concrete calculator above runs this math for slabs, footings, walls, columns and round post holes.
How the math works
Choose a slab, a footing, a wall, a column or a round post hole. Steps and curbs split into parts, and each part gets its own measure.
Type the size in the fields for that shape. The label beside each field names its unit, so feet and inches never mix.
Flat shapes multiply length by width by thickness. Round shapes use the circle area times the depth.
Length x Width x ThicknessDivide the cubic feet by 27. One cubic yard holds 27 cubic feet, and yards are the unit that yards and trucks sell by.
Cubic feet / 27Add the extra percent. Ten percent covers spills, uneven soil and forms that bow out a little on pour day.
Divide the order volume by the yield of each bag size and round up. An 80 lb bag yields 0.60 cubic feet and a 60 lb bag yields 0.45.
Cubic feet / Bag yieldA 10 by 10 foot slab, 4 inches thick
Here is the same job on a breakdown slip, the way the tool builds it.
Read the slip top to bottom. The raw volume is the bare shape. The extra line is the safety margin. The order total is the number you give the store or the yard. One ready mix truck carries this load with ease, while 62 big bags ride home only in a strong truck or across two trips.
Each shape, explained
Every shape in this concrete calculator follows the same volume rule. Only the measuring changes. Find your shape below, measure the way it says, and tap a card to load that shape in the tool.
Patios, driveways, sidewalks and shed bases. Four inches suits walks and patios. Six inches suits driveways that carry cars.
Key measure: thickness in inchesTap to load in the calculatorA strip of concrete under a wall or a post line. Deck footings and fence lines use this shape. Type the full run length in feet.
Key measure: width and depth in inchesTap to load in the calculatorGarden walls and stem walls. The form stands up, so the height drives the volume. Tall thin walls drink concrete fast, so check the bag count before a hand mix.
Key measure: thickness in inchesTap to load in the calculatorA round tube of concrete inside a cardboard form. Common tube sizes are 8, 10 and 12 inches. Type the height in feet and the number of columns.
Key measure: tube diameter in inchesTap to load in the calculatorA round hole in the ground with a post in the middle. The tool counts the full hole, and the extra percent covers the space the post takes.
Key measure: diameter and depth in inchesTap to load in the calculatorTreat each step as its own box and add the boxes together. Measure the tread depth and the riser height on each step.
Key measure: each part on its ownA flat gutter pan with a raised curb behind it. Run the pan as a slab and the curb as a low wall, then add both numbers.
Key measure: two boxes, added togetherCurb and gutter, split into two boxes
A curb and gutter is one pour with two shapes. The gutter pan lies flat in front, and the curb stands behind it. Treat the pour as two boxes and the math stays simple. Curbs and gutters repeat the same section along the street.
Box one is the gutter pan. Run the Slab shape with the run length, a pan width of 18 inches and a thickness of 6 inches. Those sizes match the standard street section in most city specs.
Box two is the curb. Run the Wall shape with the same length, a height of 12 inches and a thickness of 6 inches. The height counts the visible face plus the part below the pan line.
Street curbs come in two profiles. A barrier curb stands upright and blocks wheels. A rolled curb slopes, so cars can climb it at a driveway cut. Both profiles pour as the same two boxes.
Add both results. A 50 foot run at those sizes lands near 2.3 cubic yards before extra. Add your extra percent on the total.
Rebar and mesh inside the slab
Steel inside the slab carries the stretch loads that concrete handles poorly. It holds cracks tight and keeps the surface level when the ground moves.
The standard grid
Most home slabs use number 4 bar, a half inch thick. Space the bars about 18 inches apart each way as a planning grid.
Count the bars
Bars each way equals the span in inches divided by the spacing, plus one. A 12 foot slab at 18 inches takes 9 bars each way. Stop every bar about 3 inches short of the edges, so soil never touches the steel.
When you need more steel
Driveways under heavy trucks, RV pads and slabs over 6 inches thick earn a tighter grid at 12 inches apart.
When mesh is enough
Wire mesh suits light slabs that carry people only. Lift the mesh into the middle of the slab. Mesh flat on the ground does almost nothing.
Mix, weather and curing
The mix, the weather and the first week of care decide how a slab cures.
The classic mix
The correct mix for most home pours follows the 1 2 3 rule. One part cement, two parts sand and three parts gravel, all by volume. It lands near 3,000 psi, plenty for slabs, walks and posts.
Water discipline
Keep the water to cement ratio near 0.50 by weight. A soupy mix pours easy and cures weak. Add water in small splashes only.
Best pour weather
Pouring concrete goes best in mild weather, roughly 50 to 90 degrees. Under 40 degrees, fresh concrete can freeze and lose near half its strength. Hot dry wind dries the top fast and invites cracks.
Curing timeline
How long the cure takes sets the schedule. Keep the surface moist for the first 7 days. Feet can walk it after 24 to 48 hours. It reaches about 70 percent of full strength by day 7 and full strength at 28 days. Cars wait at least a week, longer in cold weather.
Concrete numbers, in one place
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 cu ft | 90 |
| 50 lb | 0.375 cu ft | 72 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.60 cu ft | 45 |
Common mistakes
A short pour leaves a weak joint where the second batch meets the first. Keep the extra near 10 percent for ground pours.
Past 2 yards, mixing eats hours and the cost per yard climbs past the truck price. Check both cost routes in the result first.
The label beside each field names its unit. Lengths go in as feet and thickness goes in as inches on this page.
Area covers a surface and volume fills a space. A slab needs both numbers, and the tool keeps them apart for you.
How to buy for your pour
The result moment is the buying moment. Use the numbers from this concrete calculator at the store counter or on the phone with the yard. Two routes cover almost every job.
Option A: The bag route
- Read the yield on the bag. Yields run 0.30, 0.375, 0.45 and 0.60 cubic feet by size.
- Pick a bag size your back can lift all day.
- Keep the bags dry in the truck and off the ground at home.
- Wet bags harden in the sack and turn into waste.
Option B: The ready mix truck
- Many yards set a minimum order size. Ask about it first.
- Small loads can carry a short load fee. Ask before you book.
- Book the truck for a time when your forms, tools and help stand ready.
- A waiting truck can add fees, so have the crew set before it rolls in.
Measure the area first with the square footage calculator when the shape runs odd. Count a bag only job on the concrete bag calculator, where the carry weight sits front and center. For the stone base under a slab, the gravel calculator gives tons and bags in the same style.
Questions, answered
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