How Many Bags of Concrete Do You Need?
Calculate the number of 60-lb or 80-lb concrete bags for posts, slabs, and small projects.
Bag Concrete vs Ready-Mix
For projects under 1/2 cubic yard (about 14 cu ft / twenty-three 80-lb bags), bag concrete is practical. Above that, order ready-mix from a concrete company — it is cheaper per yard, stronger, and saves hours of mixing. The break-even is roughly 30 bags: at that point, a 1-yard ready-mix delivery ($150-180) is both cheaper and easier.
Mixing Tips
Use a wheelbarrow or mixing tub. Add water gradually — too much water weakens concrete dramatically. The mix should hold its shape when squeezed (like wet sand). One 80-lb bag needs about 3.5 quarts (0.875 gallons) of water. Mix for at least 3 minutes until uniform in color. Use within 90 minutes of mixing.
For fence post holes, you can pour dry bag concrete directly into the hole around the post and add water on top — it cures underground. This "dry pour" method is faster than pre-mixing, though pre-mixed is stronger. For structural posts (deck, pergola), always pre-mix.
