You plan to repaint or install new tile in your pool and the question is how much paint or tile do you need? To calculate these amounts, you'll first need to know the surface area of your pool. Even if your pool is irregular in shape, you can calculate the surface area using estimations and some basic mathematics.
Things You'll Need
 Retractable metal tape measure
Measuring Your Pool

Measure the size of your pool in feet. For rectangular pools, measure the length and width. For circular pools, measure the diameter. If your pool is irregular in shape, imagine it bordered by a rectangle and measure the length and width of that.

Extend the tape measure through the water to the bottom of the pool and read the measurement on the tape at the level of the top of the wall. Record the depth in feet. If your pool depth varies, measure once at the shallowest and once at the deepest point. Find the average depth by adding the two numbers together and dividing by two.

Calculate the surface area of the bottom of the pool. For rectangular pools multiply the length by the width; for circular pools, half the diameter and multiply that number by itself, then multiply by 3.14, or pi if you have a scientific calculator. For irregularshaped pools, calculate the area from the rectangle measurements and then estimate by eye how much of the rectangle your pool would cover. If your pool covers 80 percent of the rectangle, your pool area will be 80 divided by 100, multiplied by the area of the rectangle.

Calculate the surface area of your pool walls. For rectangular pools, add the length and width, and multiply by twice the depth. For circular pools, multiply the diameter by the depth, then multiply by 3.14, or pi. For irregularshaped pools, either estimate by using the calculation for a rectangular pool, or measure the top rim of the pool and multiply that length by the average depth.

Add the surface area of the pool bottom and the surface area of the pool walls. This will give you the total pool area in square feet.
References
 Photo Credit Pool image by Enrico Buchs from Fotolia.com measure one image by Kimberly Reinick from Fotolia.com calculating image by timur1970 from Fotolia.com Luxury swimmingpool with orange parasol and comfortable bed image by Ivonne Wierink from Fotolia.com