Whether planning something small (like steps) or large in scope (like a patio) calculations for concrete projects of any size are basically the same. Concrete is measured in cubic yards and project length, width, and depth determine the amount needed.
For any size project, you need to multiply the length by width (to find square feet), multiply that by depth or thickness (for cubic feet) and divide by 27 (the number of cubic feet in a cubic yard) to determine how much concrete is required.
To avoid running short mid-project, always plan 10% more to compensate for below-grade variations and waste. If math is not your strength, you don't have to remember the formulas. Do these conversions using an inexpensive feet-inch-fraction "project" calculator.
Concrete is a mix of sand, gravel, water, and portland cement and is available three ways: separate ingredients that you mix, pre-mixed (just add water) and ready-mixed (delivered by truck).
For smaller projects, pre-mixed bags are great cost-wise. They are available in two sizes: 60-pound bags which on average costs $1.35 to $1.80 and 90-pound which costs $2.00 to $2.30.
However, there is a "point of diminishing returns" where the number of bags you physically need to prepare for larger projects simply overwhelms the economy of mixing it up yourself.
Do the math: A 10x10x6 patio needs 1.85 cubic yards of concrete. Using 60-pound bags (yield: one-half cubic foot per) it requires preparing 100 bags. You may want to consider ready-mix.
Cost vs. Value
Work-wise, ready-mix is a no-brainer. But what about cost-wise?
Using 60-pound bags, concrete for a patio this size ranges from $135 to $180. You'll need to rent a mixer too which adds another $40 to $60 per day.
Conversely, while calling in ready-mix reduces work, it also boosts the cost. Each cubic yard costs about $65. However, a fully loaded cement truck holds 10 cubic yards. A partial "short" load costs $15 to $20 extra for every cubic yard less than a full load.
So, our 1.85 cubic yard patio first requires purchasing two cubic yards of ready-mix (2 X $65 = $130), plus another $135 for the "short load" (10 yards full load minus 2 yards = 8 yards "short" X $17 per yard average) and now totals around $265. With assorted fees added (environmental impact, fuel surcharge, sales tax, etc.) the final cost is about $295.
Not bad. For this project, ordering ready-mix would cost from $55 to $120 more than pre-mixed - depending on where you bought your bags and rented your mixer.
Conclusion: Probably worth it and certainly a lot easier on the back.
Calculation Made Easy
A tool called ProjectCalc ® Plus makes it easy. A few keystrokes launch hundreds of home project formulas that figure material and costs with pinpoint accuracy. To estimate: enter dimensions, push "convert" and "type of material" to learn how many bricks, blocks, board feet, or cubic yards are needed. Add the price per unit and the project cost is displayed. Helps "square up" concrete forms too. You can use it at hardware stores and home centers by Calculated Industries. You can find out more by calling 1-800-854-8075 or visiting www.calculated.com.
Concrete was used as a building material as far back as ancient Rome. Projects that are done right will last a long time.
- Concrete "cures" chemically over days versus drying. To prevent cracking, cover with plastic or damp fabric.
- Additives strengthen, add color and/or speed cure time. Available for both pre-mixed bags and ready-mixed.
- Concrete may require added supports for strength like rebar, or wire mesh. Discuss project needs with a pro.
- Cement is caustic. Wear eye protection, gloves and wash off skin immediately. Do not breathe dust.
- Smooth concrete is not always best. Leave the top of footings and foundations rough for a better bond with mortars. Use a broom to texture walks/steps for safety.
- Concrete strength is measured in PSI (pounds per square inch) indicating weight it can support or bear.
- Strength is increased by adding more cement in the mix.
- Basic ready-mix is a "6 bag mix" providing 2000 PSI. For each bag added to strengthen mix: $5.00 (average).
- Weekend ready-mix delivery costs 5% to 10% more.
- Ready-mix companies allow four to five minutes for each yard delivered. After that, the clock ticks at $2 per minute.
- If concrete has to be pumped from a truck to a site, there are additional costs and charges.
- Rather than ready-mix truck returning/disposing of order overages, plan ahead for additional use such as fence posts, flag pole, and stepping stones.
- One cubic yard of ready-mix yields nine contractor-size wheelbarrows of concrete.
- Many tool rental companies offer ready-mix concrete with small minimums and reduced prices. "Spin tank" trailer you rent holds one cubic yard (must have a 3/4 ton truck or larger to haul it). Average cost: One cubic yard - $75.00, 1/2 - 65.00, 1/3 - 55.00.