How many bags of concrete for a 4-inch slab 20 × 15 ft?

Short answer: a 20′ × 15′ slab at 4" thick needs 100 cubic feet (3.7 cubic yards) of concrete. In bags, that's ~225 bags of 60 lb Quikrete or ~167 bags of 80 lb. Add 10% waste and order 248 (60 lb) or 184 (80 lb). Practically, this size pour should be a ready-mix truck (4 yd minimum), not bags — the math below explains why bags only make sense up to about 1.5 cubic yards.

The formula

Volume (cubic feet)  =  Length × Width × Thickness

  All in feet. For thickness:
    3" = 0.250 ft
    4" = 0.333 ft
    5" = 0.417 ft
    6" = 0.500 ft

Cubic yards          =  Cubic feet / 27

Bags needed          =  Cubic feet / yield-per-bag
Yield (60 lb bag):     0.45 ft³
Yield (80 lb bag):     0.60 ft³
Yield (90 lb bag):     0.675 ft³

Worked example: 20′ × 15′ × 4"

Volume:
  20 × 15 × 0.333  =  100 cubic feet

Cubic yards:
  100 / 27         =  3.7 cu yd

Bags (60 lb @ 0.45 ft³ yield):
  100 / 0.45       =  222 bags
  + 10% waste      =  244 bags  →  order 250

Bags (80 lb @ 0.60 ft³ yield):
  100 / 0.60       =  167 bags
  + 10% waste      =  184 bags  →  order 190

Bags (90 lb @ 0.675 ft³ yield):
  100 / 0.675      =  148 bags
  + 10% waste      =  163 bags  →  order 170

Why bags don't make sense at this size

Bags option:

  244 bags × 60 lb     = 14,640 lb of dry mix to handle
  244 bags × $5.50     = $1,342 in mix
  Mixing time:           ~3 minutes per bag with a tow-behind mixer
                         × 244 bags = 12 hours of mixing alone
  Crew of 4:             still 3+ hours just for the mix
  Plus pour time, finishing, cleanup, mixer rental.

Ready-mix option:

  3.75 cu yd × $185     = $694 in concrete
  Short-load fee:        $50–100 (under-4-yd surcharge)
  Delivery + chute:      $50–150
  Total:                 ~$850–950
  Pour time:             45 minutes from truck arrival
  Crew of 3:             pour, screed, float in 90 minutes
  Cleanup:               15 minutes (truck cleans itself)

Verdict: ready-mix saves ~$400 and ~10 hours.

Order 4 yd to be safe.

Slab thickness reference

3" — sidewalks, foot-traffic-only patios. Not for vehicles.
4" — standard residential interior slabs, sidewalks, light patios — this article's reference.
5" — driveways for cars and light trucks.
6" — heavy driveways, garage floors, RV pads, light commercial.
8"+ — shop floors with forklifts, equipment pads, occasional truck loads.

The math at other dimensions (quick reference)

Slab dimensions and 4"-thick yardage:

10 × 10 × 4"   =  33.3 ft³  =  1.23 cu yd  (bag job — 75 × 60lb)
10 × 12 × 4"   =  40.0 ft³  =  1.48 cu yd  (still bag-feasible)
12 × 12 × 4"   =  48.0 ft³  =  1.78 cu yd  (truck or trailer mix)
12 × 14 × 4"   =  56.0 ft³  =  2.07 cu yd  (truck recommended)
15 × 15 × 4"   =  75.0 ft³  =  2.78 cu yd  (truck)
20 × 15 × 4"   =  100   ft³  =  3.70 cu yd  (truck — this article)
20 × 20 × 4"   =  133.3 ft³  =  4.94 cu yd  (full truck)
24 × 24 × 4"   =  192   ft³  =  7.11 cu yd  (truck + maybe second batch)
30 × 30 × 4"   =  300   ft³  =  11.11 cu yd (definitely truck, plan crew)

Rule: > 1.5 cu yd, call a truck.

Bag-yield reality vs the label

Mix design (PSI) for residential slabs

2,500 PSI — minimum for residential. Some bag products are this strength.
3,000 PSI — the "default" ready-mix order for slabs and footings.
4,000 PSI — driveways, garages, anything exposed to vehicles. Spend the extra $5/yd.
4,500–5,000 PSI — commercial, heavy loads, freeze-thaw exposure.
3/4" aggregate — default for slab work.
4–6% air entrainment — mandatory for outdoor slabs in freeze-thaw climates.

Reinforcement for a 20 × 15 × 4" slab

Option 1 — Wire mesh (cheapest, weakest):
  6×6 W2.9×W2.9 sheet (10ga), 5'×10' rolls or 4'×8' sheets
  Coverage: 50 sq ft per 5'×10' sheet
  20×15 = 300 sq ft  →  6 sheets, $15 each = $90

Option 2 — #3 rebar @ 18" o.c. each way (residential standard):
  Rebar runs:  20'/1.5' = 14 + 15'/1.5' = 11 = 25 bars
                 25 × 20'  + 25 × 15'  ≈  300 LF (mixed)
  At ~$0.40/LF, $120

Option 3 — Macro fiber (poly fibers in mix):
  Add at the truck: $8–15/yd surcharge
  4 yd × $12 = $48
  Cheaper, faster — equivalent to mesh for non-structural slabs.

Option 4 — #4 rebar @ 12" o.c. each way (heavy/garage):
  20'/1' = 21 + 15'/1' = 16 = 37 bars total
  ~600 LF × $0.50/LF = $300
  Use for 5–6" thick slabs supporting cars or shop equipment.

Common pour-day mistakes

Ordering 3.5 yd to save money. Truck arrives, you're 0.3 yd short, no second truck available — pour stops mid-screed and you have a cold joint.
Pouring on frozen subgrade. Concrete loses heat to the ground, plastic-stage cracking begins before final set.
No control joints. Concrete cracks; the only question is where. Cut joints at 1/4 the slab thickness, max 24× thickness apart (so for 4": cut 1" deep at 8' max spacing).
Skipping vapor barrier on interior slabs. 6 mil poly under the slab. Otherwise the slab wicks moisture forever.
Trowelling air-entrained concrete too aggressively. Drives the air out, defeats the purpose. One float pass plus a broom finish is plenty for outdoor.

Run it on your phone

The BuilderCalc app handles slab, footing, pier, and column volumes — with bag count by 60/80/90 lb, cubic-yard rounding, and the bag-vs-truck threshold built in. 100% offline. Free on the App Store and Google Play.

Note: Local building codes and engineered drawings override these defaults. For structural slabs and engineered loads, defer to the engineer of record.

The formula

Volume (cubic feet)  =  Length × Width × Thickness

  All in feet. For thickness:
    3" = 0.250 ft
    4" = 0.333 ft
    5" = 0.417 ft
    6" = 0.500 ft

Cubic yards          =  Cubic feet / 27

Bags needed          =  Cubic feet / yield-per-bag
Yield (60 lb bag):     0.45 ft³
Yield (80 lb bag):     0.60 ft³
Yield (90 lb bag):     0.675 ft³

Worked example: 20′ × 15′ × 4"

Volume:
  20 × 15 × 0.333  =  100 cubic feet

Cubic yards:
  100 / 27         =  3.7 cu yd

Bags (60 lb @ 0.45 ft³ yield):
  100 / 0.45       =  222 bags
  + 10% waste      =  244 bags  →  order 250

Bags (80 lb @ 0.60 ft³ yield):
  100 / 0.60       =  167 bags
  + 10% waste      =  184 bags  →  order 190

Bags (90 lb @ 0.675 ft³ yield):
  100 / 0.675      =  148 bags
  + 10% waste      =  163 bags  →  order 170

Why bags don't make sense at this size

Bags option:

  244 bags × 60 lb     = 14,640 lb of dry mix to handle
  244 bags × $5.50     = $1,342 in mix
  Mixing time:           ~3 minutes per bag with a tow-behind mixer
                         × 244 bags = 12 hours of mixing alone
  Crew of 4:             still 3+ hours just for the mix
  Plus pour time, finishing, cleanup, mixer rental.

Ready-mix option:

  3.75 cu yd × $185     = $694 in concrete
  Short-load fee:        $50–100 (under-4-yd surcharge)
  Delivery + chute:      $50–150
  Total:                 ~$850–950
  Pour time:             45 minutes from truck arrival
  Crew of 3:             pour, screed, float in 90 minutes
  Cleanup:               15 minutes (truck cleans itself)

Verdict: ready-mix saves ~$400 and ~10 hours.

Order 4 yd to be safe.

Slab thickness reference

3" — sidewalks, foot-traffic-only patios. Not for vehicles.
4" — standard residential interior slabs, sidewalks, light patios — this article's reference.
5" — driveways for cars and light trucks.
6" — heavy driveways, garage floors, RV pads, light commercial.
8"+ — shop floors with forklifts, equipment pads, occasional truck loads.

The math at other dimensions (quick reference)

Slab dimensions and 4"-thick yardage:

10 × 10 × 4"   =  33.3 ft³  =  1.23 cu yd  (bag job — 75 × 60lb)
10 × 12 × 4"   =  40.0 ft³  =  1.48 cu yd  (still bag-feasible)
12 × 12 × 4"   =  48.0 ft³  =  1.78 cu yd  (truck or trailer mix)
12 × 14 × 4"   =  56.0 ft³  =  2.07 cu yd  (truck recommended)
15 × 15 × 4"   =  75.0 ft³  =  2.78 cu yd  (truck)
20 × 15 × 4"   =  100   ft³  =  3.70 cu yd  (truck — this article)
20 × 20 × 4"   =  133.3 ft³  =  4.94 cu yd  (full truck)
24 × 24 × 4"   =  192   ft³  =  7.11 cu yd  (truck + maybe second batch)
30 × 30 × 4"   =  300   ft³  =  11.11 cu yd (definitely truck, plan crew)

Rule: > 1.5 cu yd, call a truck.

Bag-yield reality vs the label

Mix design (PSI) for residential slabs

2,500 PSI — minimum for residential. Some bag products are this strength.
3,000 PSI — the "default" ready-mix order for slabs and footings.
4,000 PSI — driveways, garages, anything exposed to vehicles. Spend the extra $5/yd.
4,500–5,000 PSI — commercial, heavy loads, freeze-thaw exposure.
3/4" aggregate — default for slab work.
4–6% air entrainment — mandatory for outdoor slabs in freeze-thaw climates.

Reinforcement for a 20 × 15 × 4" slab

Option 1 — Wire mesh (cheapest, weakest):
  6×6 W2.9×W2.9 sheet (10ga), 5'×10' rolls or 4'×8' sheets
  Coverage: 50 sq ft per 5'×10' sheet
  20×15 = 300 sq ft  →  6 sheets, $15 each = $90

Option 2 — #3 rebar @ 18" o.c. each way (residential standard):
  Rebar runs:  20'/1.5' = 14 + 15'/1.5' = 11 = 25 bars
                 25 × 20'  + 25 × 15'  ≈  300 LF (mixed)
  At ~$0.40/LF, $120

Option 3 — Macro fiber (poly fibers in mix):
  Add at the truck: $8–15/yd surcharge
  4 yd × $12 = $48
  Cheaper, faster — equivalent to mesh for non-structural slabs.

Option 4 — #4 rebar @ 12" o.c. each way (heavy/garage):
  20'/1' = 21 + 15'/1' = 16 = 37 bars total
  ~600 LF × $0.50/LF = $300
  Use for 5–6" thick slabs supporting cars or shop equipment.

Common pour-day mistakes

Ordering 3.5 yd to save money. Truck arrives, you're 0.3 yd short, no second truck available — pour stops mid-screed and you have a cold joint.
Pouring on frozen subgrade. Concrete loses heat to the ground, plastic-stage cracking begins before final set.
No control joints. Concrete cracks; the only question is where. Cut joints at 1/4 the slab thickness, max 24× thickness apart (so for 4": cut 1" deep at 8' max spacing).
Skipping vapor barrier on interior slabs. 6 mil poly under the slab. Otherwise the slab wicks moisture forever.
Trowelling air-entrained concrete too aggressively. Drives the air out, defeats the purpose. One float pass plus a broom finish is plenty for outdoor.

Run it on your phone

Note: Local building codes and engineered drawings override these defaults. For structural slabs and engineered loads, defer to the engineer of record.

How many bags of concrete for a 4-inch slab 20 × 15 ft?

The formula

Worked example: 20′ × 15′ × 4"

Why bags don't make sense at this size

Slab thickness reference

The math at other dimensions (quick reference)

Bag-yield reality vs the label

Mix design (PSI) for residential slabs

Reinforcement for a 20 × 15 × 4" slab

Common pour-day mistakes

Run it on your phone

Related

Read the rest of the index.

One moment.

How many bags of concrete for a 4-inch slab 20 × 15 ft?

The formula

Worked example: 20′ × 15′ × 4"

Why bags don't make sense at this size

Slab thickness reference

The math at other dimensions (quick reference)

Bag-yield reality vs the label

Mix design (PSI) for residential slabs

Reinforcement for a 20 × 15 × 4" slab

Common pour-day mistakes

Run it on your phone

Related

Read the rest of the index.