Concrete Slab Calculator
A 12 × 12 ft slab 4 in thick holds 1.78 cu yd of concrete: 88 bags of 80 lb mix with 10% to spare, poured on 3.3 tons of gravel base. Set your own size, thickness or truck order below.
Your plan
- 88bags80 lb bags of concrete mix
- 3.3tonsCrushed stone base
- 4boards2x6 form boards
12 ft × 12 ft slab, 4 in thick: 1.78 cu yd, 88 bags of 80 lb mix
Example changeThickness none → 6 in: +44 bags of concrete
Shopping list
3 items- 80 lb bags of concrete mix48 cu ft + 10% = 52.8 cu ft ÷ 0.6 cu ft per bag; 7,040 lb in all88 bags
- Crushed stone base1.78 cu yd compacted × 1.85 tons per cu yd; about 2.24 cu yd loose on the truck3.3 tons
- 2x6 × 12 ft form boards4 sides of 12 ft: 1 board of 12 ft each; 4 boards round all 48 ft of edge4 boards
Bags, a mixer or a truck
Check before you build
- Slope the surface 1/4 in per foot away from the house (QUIKRETE).
- Where the slab meets the house, a garage floor or old concrete, set a felt or fiberboard isolation joint (Shaker Heights requires one).
- Stakes for the forms aren’t in the list: our sources print no spacing.
- Keep it damp or covered: QUIKRETE cures 5 days at 70 °F or warmer, longer when it's colder.
How this is calculated
Counts use your measurements and the assumptions below. References beside an item support its input values or rule, not the finished count.
What moves the answer most
- Thickness
- 4 in: walks and patios. City standards ask 6 in at a driveway apron and at least 3 1/2 in under a garage; a hot tub pad 4 in or more
Sources NDDOT standard drawing D-750-2, Sidewalk (2024); Oregon DOT standard drawing RD720 (2025); QUIKRETE project guide: Concrete Sidewalks and Small Slabs; City of Huber Heights, OH, driveway and sidewalk specifications; City of Saint Paul, MN, detached garage requirements (2015); Caldera Spas hot tub planning checklist
- Gravel base
- 4 in compacted
Sources NDDOT standard drawing D-750-2, Sidewalk (2024); City of Minot, ND, detail STR-7 Sidewalks (2020)
- Concrete
- 80 lb bags, 0.6 cu ft each
Source QUIKRETE Concrete Mix 1101 data sheet (Oct 2022)
- Control joints
- At most 8 ft apart: 1 across the length, 1 across the width; panels 6 ft × 6 ft
Sources QUIKRETE Concrete Mix 1101 data sheet (Oct 2022); NRMCA CIP 6, Joints in Concrete Slabs on Grade
Other assumptions (5)
- Extra concrete
- 10% for spills and an uneven base
- Weight of compacted base
- 1.85 tons per cu yd
- Joint depth
- 1 in: a quarter of the thickness, at least 1 in
- Form boards
- 2x6, 5 1/2 in actual, for a 4 in slab
Source NIST PS 20-25, American Softwood Lumber Standard (Jan 2025)
- Reinforcement
- None
Planner setting or your selection; no external document cited.
How each count is reached
- 80 lb bags of concrete mix48 cu ft + 10% = 52.8 cu ft ÷ 0.6 cu ft per bag; 7,040 lb in all
Sources QUIKRETE Concrete Mix 1101 data sheet (Oct 2022); Sakrete concrete calculators page
- Crushed stone base1.78 cu yd compacted × 1.85 tons per cu yd; about 2.24 cu yd loose on the truck
- 2x6 × 12 ft form boards4 sides of 12 ft: 1 board of 12 ft each; 4 boards round all 48 ft of edge
Source NIST PS 20-25, American Softwood Lumber Standard (Jan 2025)
All sources (22)
- QUIKRETE Concrete Mix 1101 data sheet (Oct 2022)
- Sakrete concrete calculators page
- WSDOT Design Manual, Exhibit 620-1 (Sep 2024)
- NIST PS 20-25, American Softwood Lumber Standard (Jan 2025)
- NDDOT standard drawing D-750-2, Sidewalk (2024)
- Oregon DOT standard drawing RD720 (2025)
- QUIKRETE project guide: Concrete Sidewalks and Small Slabs
- City of Huber Heights, OH, driveway and sidewalk specifications
- City of Saint Paul, MN, detached garage requirements (2015)
- Caldera Spas hot tub planning checklist
- City of Minot, ND, detail STR-7 Sidewalks (2020)
- NRMCA CIP 6, Joints in Concrete Slabs on Grade
- Kushlan 350DD/600DD operation and parts manual (2026)
- Kushlan 600DD wheelbarrow mixer product page
- Townsend Concrete ready-mix price list, Vacaville CA (Aug 2026)
- Cleveland Metroparks concrete contract, Medina Supply (2026)
- Quick Mix Concrete price list, Douglasville GA (2023)
- City of Shaker Heights, OH, driveway requirements (2019)
- QUIKRETE Concrete Calculator (run 13 Oct 2026)
- Martin Marietta aggregates calculator
- Vulcan Materials aggregate tonnage estimator
- Graniterock construction materials calculator
Where sources disagree (5)
Control joint spacing for a 4 in slab
- 6 ft to 8 ft · QUIKRETE Concrete Mix 1101 data sheet (Oct 2022)
- 24 to 36 × the thickness: 8 ft to 12 ft, 15 ft at most · NRMCA CIP 6, Joints in Concrete Slabs on Grade
- 4 ft to 6 ft (sidewalks) · NDDOT standard drawing D-750-2, Sidewalk (2024)
- 10 ft (4 in driveways) · City of Shaker Heights, OH, driveway requirements (2019)
8 ft: the tighter of QUIKRETE's upper limit and NRMCA's lower one, so both are met.
How deep to cut control joints
- 1/4 of the thickness, at least 1 in · NRMCA CIP 6, Joints in Concrete Slabs on Grade
- 1/3 of the thickness · NDDOT standard drawing D-750-2, Sidewalk (2024)
A quarter, with NRMCA and QUIKRETE; cutting a third is deeper and also fine.
Extra concrete over the measured volume
- 10% · Sakrete concrete calculators page
- None: 56 bags of 80 lb for 100 sq ft at 4 in, the bare volume · QUIKRETE Concrete Calculator (run 13 Oct 2026)
10%, Sakrete's: running short mid-pour costs more than a spare bag.
Gravel base under a walk or small slab
- 4 in · NDDOT standard drawing D-750-2, Sidewalk (2024)
- 4 in · City of Minot, ND, detail STR-7 Sidewalks (2020)
- 3 in · City of Huber Heights, OH, driveway and sidewalk specifications
- 6 in · Oregon DOT standard drawing RD720 (2025)
4 in, the depth two of the four standards draw; QUIKRETE only says "several inches".
Weight of a cubic yard of base rock
- 1.85 tons compacted · WSDOT Design Manual, Exhibit 620-1 (Sep 2024)
- 1.9 tons · Martin Marietta aggregates calculator
- 1.86 tons · Vulcan Materials aggregate tonnage estimator
- 2.03 tons · Graniterock construction materials calculator
1.85 tons, WSDOT's weight of compacted base; the quarries' calculators weigh rock by its source and grading.
88 bags · 3.3 tons · 4 boards
See listHow the slab’s concrete and base are worked out
The example is a plain rectangle, 12 × 12 ft, poured 4 in thick from 80 lb bags onto a compacted gravel base. Its numbers come from these rules, each with the arithmetic the planner ran.
Volume
The slab formula is length × width × thickness ÷ 12 for cubic feet, and 27 cubic feet make a yard: 144 sq ft at 4 in is 48 cu ft, or 1.78 cu yd. 4 in is what the North Dakota and Oregon transportation departments draw for a sidewalk, and the least QUIKRETE asks for a walk. Other shapes and depths are worked out in the concrete calculator.
Sources NDDOT standard drawing D-750-2, Sidewalk (2024); Oregon DOT standard drawing RD720 (2025); QUIKRETE project guide: Concrete Sidewalks and Small Slabs
Bags, with something to spare
Sakrete's calculators add 10% for spills and an uneven base. An 80 lb bag of QUIKRETE 1101 yields 0.6 cu ft once mixed. Bag counts for any job are on the how many bags of concrete do I need page, and the cubic feet in each bag on how many cubic feet in a bag of concrete.
Arithmetic 48 cu ft + 10% = 52.8 cu ft ÷ 0.6 cu ft per bag; 7,040 lb in all = 88 bags
Sources QUIKRETE Concrete Mix 1101 data sheet (Oct 2022); Sakrete concrete calculators page
Gravel base
North Dakota's sidewalk drawing and Minot's both put 4 in of compacted base under the slab. Yards sell rock by the ton, and WSDOT weighs compacted base at 1.85 tons per cu yd.
Arithmetic 1.78 cu yd compacted × 1.85 tons per cu yd; about 2.24 cu yd loose on the truck = 3.3 tons
Sources NDDOT standard drawing D-750-2, Sidewalk (2024); City of Minot, ND, detail STR-7 Sidewalks (2020); WSDOT Design Manual, Exhibit 620-1 (Sep 2024)
Form boards
The narrowest board that covers the thickness forms the edge: a 2x6, 5 1/2 in actual by the lumber standard. Lumber comes in 2 ft steps, so each side takes one board of the next length up that covers it, and two boards when it is longer than 16 ft. The 12 ft and 12 ft sides are counted one by one, so a plan is never short of four boards. The actual size of this board is on 2x6 actual size.
Arithmetic 4 sides of 12 ft: 1 board of 12 ft each; 4 boards round all 48 ft of edge = 4 boards
Source NIST PS 20-25, American Softwood Lumber Standard (Jan 2025)
Control joints
QUIKRETE cuts a joint every 6 ft to 8 ft; NRMCA allows 24 to 36 times the thickness. At 4 in both allow 8 ft, cut 1 in deep, a quarter of the slab.
Arithmetic At most 8 ft apart: 1 across the length, 1 across the width; panels 6 ft × 6 ft
Sources QUIKRETE Concrete Mix 1101 data sheet (Oct 2022); NRMCA CIP 6, Joints in Concrete Slabs on Grade
Where slab sources disagree
Concrete makers and city drawings disagree on joint spacing and on the base under a slab. The planner prints every value it found and says which one the example uses.
Control joint spacing for a 4 in slab
- 6 ft to 8 ftQUIKRETE Concrete Mix 1101 data sheet (Oct 2022)
- 24 to 36 × the thickness: 8 ft to 12 ft, 15 ft at mostNRMCA CIP 6, Joints in Concrete Slabs on Grade
- 4 ft to 6 ft (sidewalks)NDDOT standard drawing D-750-2, Sidewalk (2024)
- 10 ft (4 in driveways)City of Shaker Heights, OH, driveway requirements (2019)
8 ft: the tighter of QUIKRETE's upper limit and NRMCA's lower one, so both are met.
How deep to cut control joints
- 1/4 of the thickness, at least 1 inNRMCA CIP 6, Joints in Concrete Slabs on Grade
- 1/3 of the thicknessNDDOT standard drawing D-750-2, Sidewalk (2024)
A quarter, with NRMCA and QUIKRETE; cutting a third is deeper and also fine.
Extra concrete over the measured volume
- 10%Sakrete concrete calculators page
- None: 56 bags of 80 lb for 100 sq ft at 4 in, the bare volumeQUIKRETE Concrete Calculator (run 13 Oct 2026)
10%, Sakrete's: running short mid-pour costs more than a spare bag.
Gravel base under a walk or small slab
- 4 inNDDOT standard drawing D-750-2, Sidewalk (2024)
- 4 inCity of Minot, ND, detail STR-7 Sidewalks (2020)
- 3 inCity of Huber Heights, OH, driveway and sidewalk specifications
- 6 inOregon DOT standard drawing RD720 (2025)
4 in, the depth two of the four standards draw; QUIKRETE only says "several inches".
Weight of a cubic yard of base rock
- 1.85 tons compactedWSDOT Design Manual, Exhibit 620-1 (Sep 2024)
- 1.9 tonsMartin Marietta aggregates calculator
- 1.86 tonsVulcan Materials aggregate tonnage estimator
- 2.03 tonsGraniterock construction materials calculator
1.85 tons, WSDOT's weight of compacted base; the quarries' calculators weigh rock by its source and grading.
| Calculator | Cubic yards | 80 lb bags | 60 lb bags |
|---|---|---|---|
| This planner, 10% extra | 1.78 | 88 | 118 |
| This planner, no extra | 1.78 | 80 | 107 |
| DIYCalcs, no extra | 1.78 | 80 | 107 |
Before you pour
Concrete gives you one chance, so have the forms set and the forecast checked before the first bag goes in.
Pick the day
Sakrete 5000 Plus wants the air and the base between 40 and 90 °F while you work. QUIKRETE keeps a slab damp 5 days at 70 °F or warmer and 7 days between 50 and 70 °F. Sakrete lets people walk on it after 12 hours and cars after 72 hours. Under your plan, “When can I pour concrete in my city?” sets those temperatures against your city's monthly averages.
Sources Sakrete 5000 Plus Concrete Mix data sheet (2023); QUIKRETE Concrete Mix 1101 data sheet (Oct 2022)
Bags, a mixer or a truck
By hand, the example is 88 bags, 7,040 lb in all, each 80 lb to lift. In a rented 6 cu ft mixer it is 22 batches of 4 bags, 320 lb a batch. As ready-mix it is 1.96 cu yd. Townsend Concrete, Vacaville CA, its retail list of August 2026: short below 3 cu yd in zone 1, 5 cu yd in zones 2 to 5, 6 cu yd in zones 6 and 7, with a fee of $100 or the difference, whichever is less. This order is short in every zone.
Sources QUIKRETE Concrete Mix 1101 data sheet (Oct 2022); Sakrete concrete calculators page; Kushlan 350DD/600DD operation and parts manual (2026); Kushlan 600DD wheelbarrow mixer product page; Townsend Concrete ready-mix price list, Vacaville CA (Aug 2026); Cleveland Metroparks concrete contract, Medina Supply (2026); Quick Mix Concrete price list, Douglasville GA (2023)
Call 811 before digging out the base
A 4 in base is deep enough to reach a shallow line. Texas asks for 2 working days of notice; Pennsylvania asks 3 to 10 business days before construction.
Sources Common Ground Alliance, 811 in Texas; Common Ground Alliance, 811 in Pennsylvania
Garages and driveways need more
Check local garage and driveway requirements before using a patio plan for them. Saint Paul's garage handout asks at least 3 1/2 in of concrete with a perimeter 8 in thick, 6 in wide. Minot asks 6 in at a driveway apron, as do Rochester Hills and North Salt Lake; at that depth this planner's example would take 132 bags.
Sources City of Saint Paul, MN, detached garage requirements (2015); City of Minot, ND, garage permit handout; City of Rochester Hills, MI, driveway approach standards (2008); City of North Salt Lake, UT, drive approach requirements
Concrete slab questions
How many bags of concrete does a 12 × 12 ft slab take?
88 bags of 80 lb mix at 4 in thick, with Sakrete's 10% extra. The bare volume, 1.78 cu yd, is 80 bags; other sizes and brands are on how many bags of concrete do I need.
How many 60 lb bags does a 12 × 12 ft slab need?
118 with the 10% extra, 107 without. QUIKRETE prints 0.45 cu ft for a 60 lb bag, and every bag's yield is listed under how many cubic feet in a bag of concrete.
How much ready-mix should I order for a 12 × 12 ft slab?
1.96 cu yd: the measured 1.78 cu yd plus 10%. Ask the plant about its smallest load and any short-load fee before you book the truck; the yards for other shapes are on the concrete calculator.
Can a rented mixer make a 20 × 20 ft slab?
Yes, in 62 batches: 245 bags of 80 lb mix, 4 to a batch, 320 lb a batch on the manual's recipe, with the last batch holding 1 bag. As ready-mix the same slab is 5.44 cu yd.
How thick should a concrete patio slab be?
4 in for a patio or walk, the thickness two state transportation departments draw and QUIKRETE's minimum. Going to 6 in for a driveway apron changes the example this way: Thickness none → 6 in: +44 bags of concrete.
How far apart should the control joints in a 4 in slab be?
No more than 8 ft, where QUIKRETE's upper limit meets NRMCA's lower one. The example's joints: 1 across the length, 1 across the width; panels 6 ft × 6 ft. Each is cut 1 in deep.
How much gravel goes under a concrete slab?
The example puts 3.3 tons under the slab: 4 in compacted at 1.85 tons per cu yd. Huber Heights draws 3 in under its sidewalks and Oregon DOT 6 in.
How do I figure a monolithic slab with a thickened edge?
Count the flat slab, then add the edge. Saint Paul asks a garage slab for a perimeter 8 in thick and 6 in wide, poured in one piece with the slab. On the 12 × 12 ft example that is 48 ft of edge adding 8 cu ft, 0.3 cu yd, to the 1.78 cu yd of the 4 in slab. A pier or footing poured on its own is counted by the Sonotube concrete calculator.