Lesson S.3 · Start Here: Money Math Refresher
Square feet, gallons and checking the answer
4-minute read · No dated figures
Photo: Jan van der Wolf / Pexels, Pexels License
The short answer
For a rectangle, area is length × width, so a 20 ft × 60 ft driveway is 1,200 sq ft (example). For a circle, it's 3.14 × radius × radius. Split odd shapes into those pieces and add them up. For material, divide the area by the coverage on your product's data sheet, multiply by the number of coats, add some for waste, and check the answer a second way before you order.
Area: length × width, and split the odd shapes
- Rectangle: length × width. Example: a 20 ft × 60 ft driveway is 20 × 60 = 1,200 sq ft.
- Circle: 3.14 × radius × radius. The radius is half the distance across. Example: a round turnaround 20 ft across has a 10 ft radius, so 3.14 × 10 × 10 = 314 sq ft.
- Odd shapes: split them into rectangles and circles that don't overlap, figure each piece and add. Subtract anything inside that you won't cover, like a planter island.
Example (made-up measurements): an L-shaped driveway is a 20 × 60 ft run plus a 15 × 20 ft parking pad. 1,200 + 300 = 1,500 sq ft.
Some specs and suppliers use square yards. A square yard is 3 ft × 3 ft, or 9 sq ft, so divide square feet by 9: 1,200 sq ft ÷ 9 = about 133 sq yd.
Quantity: area ÷ coverage, times coats, plus waste
Quantity = area ÷ coverage per unit × number of coats. Then add a waste allowance and round up. Crack filling and striping often run on linear feet; the steps are the same with length in place of area, as long as the units match the data sheet.
The coverage number below is made up. Real coverage changes with the product and the surface, so use your product's data sheet, never this example.
Example (made-up numbers): 1,200 sq ft, a coverage of 100 sq ft per gallon per coat, and a job that calls for two coats.
- 1,200 ÷ 100 = 12 gallons per coat.
- 12 × 2 coats = 24 gallons.
- Add 10% for waste: 24 × 1.10 = 26.4 gallons.
- Round up to whole containers. In 5-gallon pails, 26.4 ÷ 5 = 5.28, so that's 6 pails.
The 10% is made up too. Set your own waste allowance from what past jobs actually used.
Check the answer before you order
- Estimate first. Round and do it in your head: 20 × 60 is 2 × 6 = 12 with two zeros added, so 1,200. If the calculator says 12,000 or 120, a zero slipped.
- Check the units. Feet × feet = square feet. Turn inches into feet before you multiply: 6 inches is 0.5 ft, so 20 ft 6 in is 20.5 ft, not 20.6.
- Watch the decimal point. 0.5 and 5 are ten times apart.
- Use a spreadsheet column. Put each piece's length in column A and its width in column B, and type
=A2*B2in column C. At the bottom,=SUM(C2:C10)adds up the pieces. Change one measurement and the total updates.
Your Move
Do this week
Measure a real driveway or lot and check the area two ways.
- Pick one you can see on a free map app or website that has a measuring tool: your own driveway, or a lot you've worked on.
- Split it into rectangles and circles. Measure each piece and write the numbers down.
- Figure each area, add them, and convert the total to square yards.
- Check it a second way: measure on site with a tape or wheel, or split the shape differently and add again. If the two answers aren't close, find out why before you trust either one.
Log it: Area checked two ways.
Common questions
How many square feet are in a square yard?
Nine. A square yard is 3 ft × 3 ft. Divide square feet by 9 to get square yards, or multiply square yards by 9 to get square feet.
Can I measure a job from a map instead of on site?
A map is a good start for planning. Check it on site before you order material or put a number in writing, because trees, shadows and old images can hide the real edges.
How do I use a rate in gallons per square yard?
Divide square feet by 9 to get square yards, then multiply by the rate and the number of coats. Check whether the rate is for the concentrate or the final mix with water and sand.
Free help
- Small Business Development Centers: free business consulting near you.
- SCORE: free business mentoring.
Free practice
Lesson check
5 questions on this lesson, one at a time, with the answer and the reason after each. Pass with 4 right, including every must-know question.
This is general education, not tax, legal, investment or insurance advice. Rules change and every situation is different. Check the sources below, and talk to a qualified professional before you act on it.
Sources
- America's SBDC: Find your SBDC
- SCORE: Free business mentoring
- For Construction Pros: Sealcoating mix design (2017)