Concrete Calculator: Bags vs Ready-Mix — Which Should You Use?

Concrete is concrete, right? Not when you're hauling it. The choice between bagged mix and ready-mix from a truck isn't just about cost — it's about volume, your back, and whether you can finish a pour before it sets. Here's how to decide and how to calculate what you need.

When Bags Make Sense

Bagged concrete mix (typically 60-lb or 80-lb bags) is the right call when:

• Total pour is under 1 cubic yard (27 cubic feet)
• You're doing post holes, fence footings, or a small pad
• You don't have truck access to the pour site
• You're working alone or with one helper
• You want to work in small batches

A standard 80-lb bag yields about 0.6 cubic feet of finished concrete. That means it takes roughly 45 bags to make one cubic yard.

Common bag sizes:

When to Order Ready-Mix

Call the batch plant when:

• Pour exceeds 1 cubic yard
• You need consistent strength (specified PSI mix)
• You're pouring a slab, driveway, or foundation wall
• You have 2+ helpers and can finish before it sets
• Truck access is available (chute reach or pump truck)

Ready-mix is priced per cubic yard and typically includes delivery. Short-load fees apply under 4–6 yards at most plants, so very small orders can be surprisingly expensive per yard.

Cost Comparison (Ballpark)

Prices vary by region, but the pattern is consistent:

The crossover point is usually around 1 cubic yard. Below that, bags win on cost and logistics. Above 2 yards, ready-mix wins on labor, time, and consistency.

How to Calculate Volume

Concrete volume is length × width × depth (all in feet):

Slab example: 10 ft × 8 ft × 4 inches deep
Convert depth: 4 ÷ 12 = 0.333 ft
Volume: 10 × 8 × 0.333 = 26.7 cubic feet
Cubic yards: 26.7 ÷ 27 = 0.99 cu yd → borderline; bags or short-load ready-mix

Post hole example: 12-inch diameter hole, 3 ft deep
Radius: 0.5 ft
Volume: π × 0.5² × 3 = 2.36 cu ft per hole
Six fence posts: 2.36 × 6 = 14.2 cu ft → 24 bags (80-lb)

Footing example: 16 inches wide × 8 inches deep × 40 ft long
0.167 ft × 0.667 ft × 40 ft = 4.45 cu ft — bags, easily

Bag Calculation Step by Step

1. Calculate volume in cubic feet
2. Divide by bag yield (0.6 cu ft for 80-lb bags)
3. Round up
4. Add 5% for spillage and uneven subgrade

For 26.7 cu ft: 26.7 ÷ 0.6 = 44.5 → 45 bags + 5% = 48 bags

Tips for Bag Mixing

• Rent a mixer for anything over 15 bags — mixing by hand in a wheelbarrow gets old fast
• Use the ratio on the bag: typically about 3 quarts of water per 80-lb bag (follow the label)
• Don't add extra water to make it easier to pour — it weakens the concrete
• Pour and finish within 30–45 minutes of mixing in hot weather

// TODO: affiliate link — Kushlan 3.5 cu ft wheelbarrow mixer handles 2 bags at a time and is worth every penny on a 40+ bag pour.

Tips for Ready-Mix

• Schedule the truck for early morning in summer
• Have at least 3 people for anything over 2 yards
• Pre-place screed boards and have your bull float ready before the truck arrives
• Know your chute reach — trucks can't always get close to backyard pours (pump trucks cost extra)

PSI: What Strength Do You Need?

• 2,500 PSI — fence posts, light footings, non-structural pads
• 3,000 PSI — patios, sidewalks, driveways (most common residential)
• 4,000 PSI — garage floors, heavy loads

Bag mix is usually rated at 4,000 PSI when mixed correctly. Ready-mix lets you specify exactly.

The Concrete Calculator handles slabs, footings, and post holes — output in cubic feet, cubic yards, and bag count. It'll tell you when you've crossed into ready-mix territory.

Quick Decision Guide

• Under 0.5 cu yd → bags, no question
• 0.5–1.5 cu yd → bags if you have help and a mixer; ready-mix if you want it done in an afternoon
• Over 1.5 cu yd → ready-mix

Concrete doesn't forgive slow work or bad math. Calculate your volume, pick your delivery method, and add 5% buffer. The Concrete Calculator does the heavy lifting on the math so you can focus on the pour.