Water Cement Ratio Calculator

1. Enter the water and choose its unit (pounds, US gallons, liters or kilograms).
2. Enter the cement and its unit (pounds, kilograms, 94 lb bags or 50 kg bags).
3. Pick non-air-entrained or air-entrained concrete for the strength estimate.
4. Read the ratio and the approximate 28-day strength, and check it against the chart below.

Here is what each result means:

The water-cement ratio is the weight of mixing water divided by the weight of cement in a concrete mix. It is one of the most important numbers in concrete, because it largely sets the strength and durability of the hardened result.

Less water relative to cement gives stronger, denser, more durable concrete, but a stiffer mix that is harder to place; more water is easier to work but weaker and more porous. The ratio is measured by weight, not volume, and it counts all the mixing water against the cement (or total cementitious material). It is a quality number, not a quantity: use the concrete mix calculator to work out how much of each material to buy.

The ratio is simply water weight divided by cement weight, both in pounds. The calculator converts gallons, liters, kilograms and bags to pounds first, then divides.

1. Convert the water to pounds (one US gallon of water is about 8.34 pounds).
2. Convert the cement to pounds (a 94 lb bag, or a 50 kg bag at about 110 lb).
3. Divide water by cement for the ratio.
4. Read the approximate 28-day strength from the ACI relationship for your air content.

Suppose a mix has 300 pounds of water and 600 pounds of cement, non-air-entrained.

Calculation: w/c = 300 / 600 = 0.50. On the ACI relationship that points to about 4,800 psi (roughly 33 MPa) at 28 days. Dropping the water to 240 pounds would give w/c = 0.40 and a much higher strength, but a stiffer mix that usually needs a water reducer to place.

The ratio, not the bag count, is what drives strength: add water on site to make placing easier and you lower the strength you paid for.

The chart below is the approximate relationship between water-cement ratio and average 28-day compressive strength from ACI 211.1. It is a planning guide, not a design value.

Slump guidance: a lower ratio gives a stiffer, lower-slump mix. Typical placing slumps are about 3 to 4 inches for slabs and pavements and 3 to 5 inches for footings and walls; reach a low ratio with a water reducer rather than by adding water.

Air Entrainment

Entrained air improves freeze-thaw durability but lowers strength at the same ratio, which is why the air-entrained column is lower.

Cement Type and Supplementary Materials

Cement class, fly ash, slag and silica fume all shift the real strength from the simple chart. Count them in the cementitious weight.

Curing

Strength assumes proper moist curing. Concrete that dries out early never reaches the figure the ratio promises.

Aggregate and Compaction

Aggregate quality, grading and full compaction all affect the result, so treat the chart value as a guide, not a guarantee.

Checking a Mix Design

Confirm the ratio of a proposed mix before you batch it, and compare it with the strength you need.

Controlling Water on Site

Check how much extra water would change the ratio before anyone adds it to make placing easier.

Meeting a Durability Limit

Many exposures cap the ratio (for example 0.45 for freeze-thaw with deicers). Use the tool to confirm the mix is under the limit.

1. Adding Water on Site

Water added at the truck to loosen the mix raises the ratio and cuts strength and durability. Use a water reducer instead.

2. Measuring by Volume

The ratio is by weight. Comparing a volume of water to a volume of cement gives the wrong number.

3. Ignoring Durability Limits

A ratio that gives enough strength can still fail a durability limit for its exposure. Check both.

4. Forgetting Supplementary Cementitious Materials

Fly ash and slag count in the cementitious weight. Leaving them out overstates the ratio.

5. Treating the Estimate as a Test Result

The chart is approximate. Real strength comes from cylinders tested to the standard, not from the ratio alone.

The ratio is exact for the weights you enter; the strength is an approximate value from a published chart, not a tested result.

For most structural concrete, a ratio of about 0.40 to 0.50 balances strength, durability and workability. Below 0.40 the mix is very strong but stiff and usually needs a water reducer; above 0.60 strength and durability fall off.

Strength falls as the ratio rises. At about 0.40 a non-air-entrained mix is near 6,000 psi; at 0.50 about 4,800 psi; at 0.60 about 3,700 psi. Extra water leaves pores that weaken the hardened concrete.

Divide the weight of mixing water by the weight of cement, both in pounds. For 300 pounds of water and 600 pounds of cement the ratio is 0.50. The calculator converts gallons, liters, kilograms and bags for you.

About 0.57 for non-air-entrained concrete on the ACI chart, or lower for air-entrained. Real mixes aim a little below the chart value to allow for site conditions, so around 0.50 to 0.55 is common for 4,000 psi.

Adding water to make concrete easier to place raises the water-cement ratio and lowers the strength and durability you ordered. Use a water-reducing admixture instead, which improves workability without extra water.

By weight. You compare the pounds of water to the pounds of cement. Because water and cement have very different densities, a volume comparison gives the wrong ratio.

It depends on the exposure. Durability limits in ACI 318 cap the ratio for conditions like freeze-thaw (often 0.45) or exposure to deicers and sulfates. Check the limit for your exposure, not just the strength.

The ratio is calculated the same way, but air-entrained concrete is a little weaker at the same ratio, so the strength estimate is lower. It is used where freeze-thaw durability matters.

No. The calculation runs entirely in your browser and nothing you enter is stored or sent anywhere.

• American Cement Association (Portland Cement Association) (American Cement Association; water-cement ratio and strength).
• National Ready Mixed Concrete Association (NRMCA; concrete proportioning and durability).
• QUIKRETE Concrete Mix product data (QUIKRETE; mixing water guidance).
• QUIKRETE concrete and bag calculator (QUIKRETE; volume and yield method).