How we calculate this
- Bag yields are from the Quikrete Concrete Mix data sheet: 40 lb 0.30 cu ft, 50 lb 0.375, 60 lb 0.45, 80 lb 0.60 and 90 lb 0.675. Other brands are close but not identical, so check the bag.
- Mixing water uses Quikrete’s range per bag (6 to 9 pints for an 80 lb bag).
- Forms use the smallest board at least as tall as the slab is thick (a 2x4 is only 3 1/2 in tall, a 2x6 is 5 1/2 in, per NIST PS 20), in stock lengths. Stakes go roughly every 3 ft plus the corners.
- Rebar runs both ways at your spacing, cut from 20 ft sticks. Bars stop 3 in from the edges, the code’s cover for concrete cast against the earth (R403.1.3.5.3). Where a bar is longer than a stick, the pieces overlap 30 in, the code’s lap for Grade 60 #4 bars (Table R608.5.4(1); Grade 40 needs 20 in). Both rules are written for footings and walls; the IRC sets no rebar spacing for a shed slab, so the 18 in default is a placeholder to confirm with your building department.
- Wire mesh comes in 5 × 10 ft sheets with a 6 in grid. Sheets overlap one grid square plus 2 in (ACI 439.5R, as published by the American Society of Concrete Contractors), so each sheet covers about 40 sq ft.
- Ready-mix is the same volume plus waste, rounded up to a quarter yard. It’s shown as an alternative and left out of the total.
Worked example: how much concrete for a 10 × 12 shed slab?
A 12 ft × 10 ft shed slab, 4 in thick, on 4 in of gravel:
- Volume: 12 × 10 × 4 ÷ 12 = 40 cu ft, or 1.48 cu yd.
- With 10% waste: 44 cu ft ÷ 0.6 = 74 bags of 80 lb. That’s 5,920 lb to haul and 56 to 83 gal of mixing water.
- Or ready-mix: 1.75 cu yd.
- Gravel: 1.7 cu yd. Forms: 4 2x6 boards and 19 stakes. Rebar: 11 sticks at 18 in.
- Joints: 2 control joints, splitting the slab into 4 panels of about 6 ft × 5 ft.
Safety and when to call a pro
Wet concrete is caustic. Quikrete’s data sheet warns it causes severe skin burns and eye damage and that the dust can harm your lungs: wear impervious gloves (such as nitrile), eye protection and a mask. A slab this size is a lot of material to mix and finish before it sets, so line up helpers or a mixer.
This calculator is for a simple shed floor. Garages, house additions and slabs that carry walls or heavy equipment need footings and reinforcement designed to your local code. A shed over 200 sq ft usually needs a building permit.
Numbers this calculator uses
Every value comes from the data files behind this page. Anything marked PLACEHOLDER hasn’t been checked against a source yet, and you can override it in the calculator where it matters.
| Number | Value | Source |
|---|---|---|
| Cubic feet in a cubic yard | 27 | Unit definition |
| Minimum slab-on-ground thicknessR506.1. | 3 1/2 in | 2024 IRCchecked 2026-10-30 |
| Gravel base under a below-grade slabR506.3.2: not required on well-drained Group I soils. | 4 in | 2024 IRCchecked 2026-10-30 |
| Largest one-story detached building exempt from a permitR105.2. Local rules can be stricter. | 200 sq ft | 2024 IRCchecked 2026-10-30 |
| Rebar grid spacing (default)Not set by the IRC for a shed slab. Ask your building department or follow your plans. | 18 in | PLACEHOLDER |
| Rebar distance from slab edgesR403.1.3.5.3: 3 in of cover for concrete cast against the earth. That rule is written for footings; we use it for slab edges too. | 3 in | 2024 IRCchecked 2026-10-30 |
| Rebar stick length#4 rebar is sold in 20 ft sticks (also 10 ft). | 20 ft | The Home Depotchecked 2026-10-30 |
| Rebar overlap where bars joinTable R608.5.4(1): #4 bars lap 30 in for Grade 60 steel, 20 in for Grade 40. We use 30 in so it works for either. The table is written for concrete walls. | 30 in | 2024 IRCchecked 2026-10-30 |
| Wire mesh sheet width | 5 ft | Kuhlmanchecked 2026-10-30 |
| Wire mesh sheet length | 10 ft | Kuhlmanchecked 2026-10-30 |
| Wire mesh grid spacing | 6 in | Kuhlmanchecked 2026-10-30 |
| Wire mesh lap beyond one grid squareACI 439.5R-18 section 6.4, as quoted by ASCC: lap one square plus 2 in, so 8 in for 6 x 6 mesh. | 2 in | American Society of Concrete Contractorschecked 2026-10-30 |
| Form stake spacing | 3 ft | PLACEHOLDER |
| Bagged paver base volume | 0.5 cu ft | The Home Depotchecked 2026-10-31 |
| Waste factor for concrete and gravel (default; you can change it)Covers uneven ground, spills and form bulge. Gravel also settles when compacted. | 10% | PLACEHOLDER |
| Concrete mix yield per bag | 40 lb: 0.3 cu ft; 50 lb: 0.375 cu ft; 60 lb: 0.45 cu ft; 80 lb: 0.6 cu ft; 90 lb: 0.675 cu ft | The Quikrete Companieschecked 2026-10-30 |
| Thinnest pour the mix is designed for | 2 in | The Quikrete Companieschecked 2026-10-30 |
| Control joints at most everyQuikrete: every 6 to 8 ft. | 8 ft | The Quikrete Companieschecked 2026-10-30 |
| Control joint depth | 1 in | The Quikrete Companieschecked 2026-10-30 |
| Form board heights | 2x4: 3 1/2 in; 2x6: 5 1/2 in; 2x8: 7 1/4 in; 2x10: 9 1/4 in; 2x12: 11 1/4 in | National Institute of Standards and Technologychecked 2026-10-30 |
Sources
| Source | What we use from it | Last checked |
|---|---|---|
| QUIKRETE Concrete Mix No. 1101 data sheet (revised October 2022)The Quikrete Companies · manufacturer spec | Bag yields (40 lb 0.30 cu ft, 50 lb 0.375, 60 lb 0.45, 80 lb 0.60, 90 lb 0.675), mixing water per bag, minimum pour thickness of 2 in, control joints every 6 to 8 ft, fence post hole size (3x the post width, 1/3 of the post's length deep, 6 in of mix under the post). | 2026-10-30 |
| 2024 International Residential Code, Chapter 5 (Floors): R506 and R507International Code Council · building code | Slab thickness (R506.1), base course (R506.3.2), vapor retarder (R506.3.3), deck beam spans (Tables R507.5(1)-(4)), deck joist spans and cantilevers (Table R507.6), decking joist spacing and fastening (R507.7). | 2026-10-30 |
| 2024 International Residential Code, Section R105.2 (Work exempt from permit)International Code Council · building code | Permit exemptions: fences up to 7 ft, retaining walls up to 4 ft (bottom of footing to top of wall, no surcharge), detached accessory buildings up to 200 sq ft, small low unattached decks. | 2026-10-30 |
| 2024 International Residential Code, Section R403.1.3.5.3 (Support and cover)International Code Council · building code | Reinforcement in concrete cast against the earth needs at least 3 in of cover (written for footings; we use it to keep slab bars 3 in from the edges). | 2026-10-30 |
| 2024 International Residential Code, Table R608.5.4(1) (Lap splice and tension development lengths)International Code Council · building code | #4 bars lap 20 in for Grade 40 steel and 30 in for Grade 60 (written for concrete walls). | 2026-10-30 |
| Voluntary Product Standard PS 20-10, American Softwood Lumber Standard, Table 3National Institute of Standards and Technology · standard | Actual (dressed, dry) sizes of nominal lumber: 1x6 is 3/4 x 5-1/2 in, 5/4 boards are 1 in thick, 2x lumber is 1-1/2 in thick, 4x4 is 3-1/2 in, 6x6 is 5-1/2 in. | 2026-10-30 |
| Southern Pine specification guidelinesSouthern Pine Council · industry guide | Dimension lumber is specified in standard lengths from 8 to 20 ft in 2 ft steps. | 2026-10-30 |
| 1/2 in x 20 ft #4 rebar (REB/4/615G4/20), Home Depot listingThe Home Depot · store listing | #4 rebar sold in 20 ft sticks of Grade 40 steel. | 2026-10-30 |
| 10-gauge wire mesh, 5 ft x 10 ft sheet, 6 x 6 in spacing, Kuhlman listingKuhlman · store listing | Welded wire mesh sold in 5 x 10 ft sheets with a 6 x 6 in grid. | 2026-10-30 |
| WWR Lap Splice Requirements (The Voice, January 2021)American Society of Concrete Contractors · industry guide | Cites ACI 439.5R-18 section 6.4: lap plain welded wire mesh one grid square plus 2 in. | 2026-10-30 |
Spotted a number that’s out of date? Tell us and we’ll recheck it and note the fix in the change log.
Frequently asked questions
How many 80 lb bags of concrete make a cubic yard?
About 45. A cubic yard is 27 cu ft and an 80 lb bag of Quikrete Concrete Mix makes about 0.6 cu ft, so 27 ÷ 0.6 = 45. With 60 lb bags (0.45 cu ft each) it’s 60. Add a little for waste.
How thick should a shed slab be?
The 2024 IRC’s minimum for a concrete slab floor is 3.5 in (R506.1). Your building department may require more for a shed that will hold a riding mower, a vehicle or heavy shelving, and a slab that carries walls needs footings designed to local code.
Do I need gravel under a concrete slab?
The code calls for a 4 in base of clean graded sand, gravel or crushed stone under slabs below grade, unless the soil is already well-drained sand or gravel (R506.3.2). Compact it before you pour.
Do I need a vapor barrier under a shed slab?
The 2024 IRC doesn’t require one under garages, utility buildings and other unheated accessory structures (R506.3.3). If you’ll store things that mind damp, you can still add 6-mil polyethylene, lapped at the joints.
Should I use bags or order ready-mix?
Compare the two lines in the cost estimate with your local prices. Bags are easy to buy but heavy: the weight to haul is shown with the results. Ready-mix plants often have minimum orders and short-load fees, so ask before you choose.
Where do control joints go?
Quikrete’s instructions call for joints about 1 in deep every 6 to 8 ft. The calculator splits each side into equal panels no more than 8 ft across and draws the joint lines on the diagram.
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