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Mixing concrete for footings and small slabs

Bagged concrete is already blended — cement, sand and gravel. You add water, not ingredients. The bag's water line wins over this page; everything here is the process around it.

Last reviewed Oct 23, 2026

The bag or tube wins

Product formulations change. If the manufacturer’s printed ratio, temperature or cure differs from this page, follow the manufacturer — and tell us so we can update the guide.

At a glance

By job

Post hole / footing

Ratio
1 × 80 lb bag
Water
2.5–3 qt (2.4–2.8 L)
Mixing
3–5 min
Working time
20–30 min
Cure
24–48 h before load

Crown the top away from the post; set the post base while the concrete is wet.

Small slab (up to about 4 × 4 ft)

Ratio
1 × 80 lb bag
Water
2.5–3 qt (2.4–2.8 L)
Mixing
3–5 min
Working time
20–30 min
Cure
Keep damp 3 days; foot traffic 48 h

One continuous pour; wire mesh sits mid-depth, not on the ground.

Patching a hollow or a broken edge

Ratio
1 bag
Water
A little stiffer than a pour
Mixing
3–5 min
Working time
15–20 min
Cure
24 h

Dampen the old concrete; do not feather the patch to zero thickness.

Choose the product

  • Use a bagged concrete mix for post holes, footings and small slabs. An 80 lb bag yields about 0.6 cu ft.
  • Use a fast-setting post mix only where the bag says so — it sets in minutes, not hours, and leaves little time to adjust a post.
  • Order ready-mix when the pour is larger than about 1 cubic yard, or when a truck can reach the work. Bagged mixing stops being practical past that.
  • Do not substitute mortar mix, sand mix or paver base for concrete.

Mix it

  1. Put about three-quarters of the water in the tub first, then add the bag. Water-first wets the powder evenly and keeps dust down.
  2. Mix for 3–5 minutes with a hoe or paddle until the color is uniform; break up dry pockets at the bottom of the tub.
  3. Add the rest of the water a splash at a time. Target thick oatmeal that holds a shovel mark — not soup.
  4. If it feels stiff, let it rest 2–3 minutes and remix briefly. Once it starts to set, do not add water.
  5. Mix only what you can place in 20–30 minutes. In heat, mix half batches.

Weather window

Temperature

40–80°F (5–27°C)

Rain

Do not place in standing water or on frozen ground. Cover the work if rain is due within 24 hours.

Watch for

  • Below 40°F the chemical set stalls; keep the work above 40°F for the first 24 hours. Never add antifreeze.
  • Above 90°F, or in strong wind, use cool water, shade the work and finish quickly — hot concrete loses strength and cracks.

Cure

  • Foot traffic after about 24–48 hours; furniture and heavy loads after 7 days.
  • Concrete keeps gaining strength for 28 days. Slabs should stay damp and shaded for the first 3 days — mist or cover them.
  • Do not set stone or build on green concrete; trapped moisture cracks both.

When it goes wrong

  • Crumbly, dusty surface

    Likely cause: Too much water, or it dried in sun and wind before curing.

    Fix: Mix stiffer; keep the work damp and shaded for 3 days.

  • Cracks across the slab

    Likely cause: Too much water, no control joint, or rapid drying.

    Fix: Use the bag ratio, add the designed control joint, and cure damp.

  • Base or post moves after setting

    Likely cause: Soupy mix, or the hardware was not braced while wet.

    Fix: Mix thick, set the base immediately and check plumb before it sets.

Safety

  • Wear gloves and eye protection. Wet concrete is alkaline and can burn skin.
  • Wear an N95 when handling dry mix or cutting concrete — silica dust is the long-term hazard.
  • Do not wash tools or dump wash water into storm drains. Let solids settle and dispose of them on site.

Sources