Should I buy 40, 60, or 80 lb bags?

Fewer, heavier bags mean fewer to open and mix, so 80 lb bags win on labor for most slabs. Choose 60 lb or 40 lb bags when you cannot lift 80 lb comfortably or need to carry them a long way — you will just buy more of them.

When should I switch from bags to a ready-mix truck?

Around 1.5 cu yd, or roughly 60–70 × 80 lb bags. Below that, bags avoid a short-load fee and the wait for a truck. Above it, mixing by hand is slow and the cold joints between batches can weaken the slab.

Does the waste allowance really matter?

Yes. A 5–10% allowance covers uneven subgrade, form deflection, and spillage. Running 2 or 3 bags short mid-pour means a cold joint or a second trip to the store, so it is cheap insurance.

Concrete Bags Calculator

Enter your pour and get how many 40, 60, or 80 lb bags of concrete mix to buy — with waste built in and a bags-vs-truck gut check.

Inputs

Pad size

ft ×

Pad size, second side

Thickness — 4″ typical for patios

Bag size

Waste

Results update as you type — no submit

80-lb bags to buy

74 bags

≈ 5,920 lb of dry mix

Concrete volume

1.6 cu yd

≈ 1.25 m³

Concrete volume1.6 cu yd

In 60-lb bags98 bags

In 40-lb bags147 bags

Slab area120 sq ft

Consider a truck. Past 1.5 cu yd (~60–70 bags), ready-mix usually beats bags on cost and cure quality.

Formula

volume = 12 × 10 × (4 ÷ 12) = 40 cu ft
bags = ceil(40 × 1.1 ÷ 0.6) = 74

Diagram — pour plan & sectionNTS

MeasureShedmeasureshed.com/calculator/concrete-bags-calculator

Concrete bags — material listUNITS: FT · SCALE: NTS

Inputs

Pad size10 × 12 ft

Thickness4″

Bag size80 lb

Waste+10%

Results

80-lb bags74

In 60-lb bags98 bags

In 40-lb bags147 bags

Concrete volume1.6 cu yd

Slab area120 sq ft

Pour plan & sectionNTS

Shopping list

☐ 74 × 80 lb bags concrete mix

Formula

volume = 12 × 10 × (4 ÷ 12) = 40 cu ft

order = 40 × 1.1 = 44 cu ft (1.6 cu yd)

bags = ceil(44 ÷ 0.6) = 74

Estimates for planning only — verify before purchaseSHEET 1 OF 1

How it works

Bagged concrete is sold by how much finished concrete a dry bag makes, not by weight of concrete. An 80 lb bag of standard mix yields about 0.60 cubic feet, a 60 lb bag about 0.45, and a 40 lb bag about 0.30. So the job is to find the volume of your pour and divide by the bag yield.

Volume is length × width × thickness. Thickness is usually in inches while the slab is in feet, so the calculator divides thickness by 12 first, which gives cubic feet. It then adds a small waste allowance — forms bow, subgrade dips, and a little always spills — and divides the padded volume by each bag size's yield. Bag counts always round up, because you cannot buy part of a bag.

The bag route is convenient for small pours, but the math turns against it fast: past roughly 1.5 cubic yards (about 60 to 70 bags), a ready-mix truck is usually cheaper per yard and pours the slab in one piece instead of dozens of hand-mixed batches.

Worked example

Pour 10 ft × 12 ft · thickness 4″ · +10% waste
volume = 10 × 12 × (4 ÷ 12) = 40 cu ft → +10% = 44 cu ft
80 lb bags = ceil(44 ÷ 0.60) = 74 · 60 lb = ceil(44 ÷ 0.45) = 98 · 40 lb = ceil(44 ÷ 0.30) = 147

Questions from the jobsite

A cubic yard is 27 cu ft. At 0.60 cu ft per 80 lb bag you need about 45 bags; at 0.45 cu ft per 60 lb bag about 60; and at 0.30 cu ft per 40 lb bag about 90 — all before any waste allowance.

SOURCES — Bag yields per QUIKRETE Concrete Mix No. 1101 & SAKRETE data sheets

Estimates for planning only — verify before purchase