Manually calculating your stain needs involves a few key factors. Here’s how to do it:

1. Calculate Total Surface Area: First, determine the total square footage of the surface you need to stain. For multiple items or complex shapes, calculate the area of each part and add them together.
2. Account for Multiple Coats: Multiply the total surface area by the number of coats you plan to apply. For example, 100 sq ft requiring two coats becomes 200 sq ft of total coverage needed.
3. Check Stain Coverage Rate: Look at the product label for the manufacturer's estimated coverage rate, usually stated in "square feet per quart" or "square feet per gallon". A common rate is around 150 sq ft per quart.
4. Adjust for Wood Porosity: If you are staining a porous wood like pine, it will absorb more stain, so you may only get 80% of the stated coverage. For dense woods like maple, you might get 120%. Adjust the coverage rate accordingly.
5. Calculate Total Stain Needed: Divide the total area to be stained (from step 2) by the adjusted coverage rate (from step 4) to find out how many quarts or gallons you need.

• Stain: A type of paint used to color wood and other materials. Unlike paint, which sits on the surface, stain is designed to penetrate the material.
• Coverage Rate: The amount of area a given volume of stain can be expected to cover, typically measured in square feet per quart or gallon.
• Wood Porosity: The degree to which wood absorbs liquids. Highly porous woods (like pine) soak up more stain than dense, less porous woods (like maple).
• Oil-Based Stain: A slow-drying stain that uses a petroleum-based solvent. It penetrates deeply and is known for its rich color.
• Water-Based Stain: A fast-drying stain that uses water as a solvent. It has low odor and is easy to clean up but may raise the wood grain.
• Gel Stain: A thick, non-drip stain that sits more on the surface of the wood. It's excellent for preventing blotching on porous woods.

"The coverage numbers on the can are just an estimate, usually for ideal conditions on a medium-density wood. The biggest factor they can't account for is your application technique. A heavy hand with a brush will use significantly more stain than wiping it on and off with a rag. For the most even and professional-looking finish, I always recommend wiping the stain on with a lint-free cloth, letting it penetrate for a few minutes, and then wiping off all the excess. This method gives you the most control and also maximizes your coverage, making your stain go further." - Professional Finisher

Staining a New Pine Deck: A deck is 12 feet by 20 feet, totaling 240 sq ft. Pine is a porous wood, and you are applying one coat.
An oil-based stain might have a stated coverage of 150 sq ft/quart, but on porous pine, you should expect closer to `150 * 0.8 = 120` sq ft/quart.
Stain needed: `240 sq ft / 120 sq ft/quart = 2` quarts. It would be wise to buy 3 quarts to have extra.

Refinishing a Maple Dining Table: The tabletop is 3 ft by 6 ft (18 sq ft), and you are applying two coats of a gel stain.
Total area to cover: `18 sq ft * 2 coats = 36 sq ft`.
Gel stain has lower coverage, about 100 sq ft/quart. Since maple is dense, you might get 120% of that (`120 sq ft/quart`).
Stain needed: `36 sq ft / 120 sq ft/quart = 0.3` quarts. A single pint (0.5 quarts) would be more than enough.

• Not Preparing the Surface: Failing to properly sand the wood to the right grit will result in uneven stain absorption and a blotchy appearance.
• Forgetting to Stir the Can: The color pigments in stain settle to the bottom of the can. If you don't stir it thoroughly before and during use, the color will be inconsistent.
• Applying Stain Too Heavily: Leaving a thick film of stain on the surface instead of wiping off the excess results in a sticky, uneven finish that takes a very long time to dry.
• Ignoring End Grain: The end grain of a board is like a bundle of straws and will soak up much more stain, appearing very dark. Always apply a wood conditioner or a thin coat of finish to the end grain first to control absorption.

• Deck and Fence Maintenance: Calculating the large quantities of stain needed for outdoor projects.
• Furniture Refinishing: Estimating the right amount of stain for chairs, tables, and dressers.
• Cabinetry and Built-Ins: Ensuring a consistent color match across a large set of kitchen cabinets or a bookshelf.
• Hardwood Floor Finishing: Budgeting for materials when staining or re-staining hardwood floors.