Behr Stain Calculator: How to Estimate

The Behr stain calculator is a tool on the Behr website that estimates how much exterior wood stain you need for a deck, fence, or siding project. You enter the surface type and its dimensions (length and width, or total square footage), and it returns the number of gallons or kits required. It also factors in the number of coats you plan to apply.

Formula

The calculator uses a simple coverage-based formula:

Stain needed (gallons) = (Length × Width × Number of Coats) ÷ Coverage per gallon

Where:

• Length (ft) – the longer dimension of the surface.
• Width (ft) – the shorter dimension.
• Number of Coats – typically 1 or 2 for most exterior stains.
• Coverage per gallon – the manufacturer’s stated rate, which is 200 ft² per gallon for most Behr exterior stains on smooth wood. Rough or weathered wood will need more stain, often 100–150 ft² per gallon.

The total area (square feet) is simply length × width. If you’re staining a fence, you only measure the flat surface you’ll actually coat, not both sides unless you plan to stain both.

Worked Example

Using the default values from the calculator above:

• Length = 40 ft
• Width = 25 ft
• Coats = 2

First, find the total square footage: 40 × 25 = 1,000 ft².

Then multiply by the number of coats: 1,000 × 2 = 2,000 ft² of total coverage needed.

Divide by the coverage rate (200 ft² per gallon): 2,000 ÷ 200 = 10 gallons.

The calculator shows 10.0 gallons, which matches. If you were applying only one coat, you’d need 5 gallons.

How the Behr Calculator Works

The official Behr stain calculator is a web-based tool that asks for the type of surface (deck, fence, siding, or other), the dimensions, and the number of coats. It then uses a coverage rate that Behr publishes for its own stain products. The rate varies by product line: for example, a solid color stain might cover 200 ft² per gallon on smooth wood, while a transparent or semi-transparent stain could cover 300–400 ft² per gallon on a smooth surface. The calculator also accounts for the fact that rough-sawn or weathered wood absorbs more stain, so it will adjust the coverage rate downward.

You can find the calculator on the Behr website under the “Project Calculator� section. It’s free to use and doesn’t require an account. You enter your measurements in feet and inches, choose the number of coats, and it returns the number of gallons and kits (a kit usually contains two 1-gallon cans) you should buy.

Choosing the Right Stain and Coverage Rate

Before using any stain calculator, you need to know what type of stain you’ll apply. The coverage rate changes based on the product’s opacity:

• Transparent stain – penetrates the wood, minimal pigment, covers roughly 300–400 ft² per gallon on smooth wood.
• Semi-transparent stain – adds some color, covers about 200–300 ft² per gallon.
• Solid color stain – acts like a paint film, covers about 200 ft² per gallon.

If your wood is rough, weathered, or very porous, the coverage will drop by 20–50%. For example, a rough cedar fence might only get 100–150 ft² per gallon with a semi-transparent stain. The Behr calculator lets you select the surface condition, but if you’re using a generic calculator, you should manually reduce the coverage rate for rough wood.

Common Mistakes and How to Avoid Them

One frequent error is measuring the wrong area. For a deck, you measure the entire deck surface, including the spaces between boards? Actually, you measure the total floor area, not just the boards. For a fence, you measure the face of the fence, not the perimeter. If you’re staining both sides, double the area.

Another mistake is forgetting the number of coats. Most exterior stains require two coats for even color and protection, especially on new wood. The calculator asks for this, but if you’re using a simple formula, don’t forget to multiply by the number of coats.

Unit mix-ups also happen. If you measure in inches, convert to feet before using the formula. 1 ft = 12 in. For example, a 20 ft × 10 ft deck is 200 ft², but if you mistakenly use 20 in × 10 in, you’ll get 1.39 ft², which is wrong.

Rounding up is always wise. Stain is sold in whole gallons, and you can’t buy a fraction of a gallon (unless you buy a quart). If your calculation says 2.3 gallons, you’ll need to buy 3 gallons. The Behr calculator does this rounding automatically.

Comparing to Other Stain Calculators

Other paint and stain manufacturers offer similar calculators, such as Sherwin-Williams and Benjamin Moore. They all use the same basic formula: area ÷ coverage rate × coats. The main difference is the coverage rate they assume for their own products. Behr’s rates are generally in line with industry standards, but you should always check the label on the specific stain you buy. The label will state the exact coverage in square feet per gallon for that product.

Some calculators also ask for the type of wood (cedar, pine, redwood) and the surface condition (new, weathered, previously stained). These factors affect the coverage rate. The Behr calculator includes these options, so it can give a more accurate estimate than a generic one.

Cost Considerations

Stain prices vary widely, but you can estimate the cost once you know the gallons needed. For example, if you need 10 gallons and a gallon costs $40, the stain alone will be $400. But don’t forget other costs: brushes, rollers, drop cloths, and possibly a power washer to clean the surface. The calculator only tells you how much stain to buy, not the total project cost.

A common misconception is that more stain is always better. Applying too much stain can cause it to pool and dry sticky, especially on horizontal surfaces. The calculator gives you the right amount for the recommended number of coats. Follow the manufacturer’s instructions for application and drying times.

Safety and Preparation

Before staining, ensure the wood is clean and dry. Most stains require the wood to have a moisture content below 15%. If you’re working with old stain, you may need to strip it off. Use protective gear: gloves, safety glasses, and a respirator if you’re working in a confined space. Stain fumes can be strong, so work in a well-ventilated area.

If you’re applying stain to a deck that will be used for food preparation (like a picnic table), make sure the stain is rated for food contact. Most exterior stains are not, so check the label. Also, keep stain away from plants and grass; it can harm vegetation. Cover nearby plants with plastic sheeting.

Frequently Asked Questions

How do I use the Behr stain calculator?

Go to the Behr website, find the “Project Calculator� under the “Tools� section, select “Stain� and then choose the surface type (deck, fence, siding). Enter the dimensions in feet and inches, select the number of coats, and click calculate. The tool will show the gallons needed and the number of kits to buy.

What is the coverage rate for Behr stain?

For most Behr exterior stains, the coverage rate is about 200 ft² per gallon on smooth wood. For rough or weathered wood, it drops to 100–150 ft² per gallon. Always check the label on the can for the exact rate for that product.

Can I use the calculator for a fence with pickets?

Yes, but you need to measure the total surface area of the pickets, not just the length of the fence. If the fence is a solid board-on-board style, you measure the entire face. If it’s a spaced picket fence, you measure the picket width and height and multiply by the number of pickets. The calculator doesn’t do this for you, so you have to compute the area manually.

Should I always buy extra stain?

It’s smart to buy 10% more than the calculated amount to account for waste, spills, and touch-ups. Since stain is sold in whole gallons, you’ll often have to round up anyway. If you have leftover stain, you can store it in a cool, dry place for future touch-ups.

Does the calculator work for interior staining?

The Behr calculator is designed for exterior surfaces, but the formula works for interior wood as well. Interior stains have similar coverage rates, but you should use the rate specified on the interior stain label. The calculator may not have an interior option, so you can use the exterior setting and adjust the coverage rate manually.

What if my deck is not a perfect rectangle?

Break the deck into smaller rectangles or triangles, calculate each area, and add them together. For example, an L-shaped deck can be split into two rectangles. The calculator only accepts a single length and width, so you’ll need to do this manually and then enter the total square footage if the calculator allows it, or use a custom calculator like the one above.

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in l | Surface Length | ft | 40 | 0 | 100000
in w | Surface Width | ft | 25 | 0 | 100000
in d | Number of Coats | - | 2 | 1 | 4
out gal | Stain Needed | gallons | l * w * d / 200 | 1
out sqft | Total Area | ft² | l * w | 0