How the bag concrete calculator works
Bag yields, memorized
80-lb bag ≈ 0.60 cu ft; 60-lb ≈ 0.45; 50-lb ≈ 0.375. A cubic yard is 27 cu ft — 45 bags of 80-lb, 60 of 60-lb. Post holes (12" tube × 3 ft) take ~2.4 bags each. These four numbers cover 90% of weekend concrete; the calculator handles the rest with the overage included.
The water discipline
Bag concrete wants ~3 quarts per 80-lb bag — the wet-soup mix that pours itself is the weak mix that cracks. Workable-and-stiff beats soupy every time; strength drops roughly 10% per extra quart. The one exception is high-early or flow formulations, which say so on the bag and mean it.
Mixing logistics
A rented mixer (~$40/day) turns 4 minutes per bag into the whole day's plan; hand-mixing in a wheelbarrow doubles the time and triples the regret past ~15 bags. Mix continuously once started — cold joints between bag batches are as real as between trucks, just smaller.