Decks wear out over time and sometimes need to be replaced. This can be a costly undertaking, so you want your estimate to come in close to the actual amount. The materials required, deck size and labor required will affect the cost. If you use wood instead of manufactured decking materials, you will spend less up front. However, you may save in the long run if you use manufactured materials because you won’t need to stain or perform much maintenance on the prefabricated decking. If your deck is large, your replacement bill will likely be large, too. If you hire someone to do the job, your cost will likely increase by at least 50 percent over the cost of doing it yourself.

Measure the size of your current deck. Measure the width and the length of the deck and multiply the two numbers. The answer gives you the area of the deck. If you are not changing the design, use the area number to calculate how much material you need. If you are replacing support beams, railings and stairs, measure how much material you need for each. If you are not changing the design, count the stair treads, measure the length of your railing and count how many support beams your current deck uses. Measure the height, length and width of the support beams to ensure you buy the correct size replacement materials.

Visit your local home improvement store or building supply store. Talk to an employee and show him your building list to determine which materials would best suit your needs. Compare prices of the materials.

Use the area of the deck to figure the cost of the materials, whether you choose wood or composite boards. Determine how many floor boards you need. Estimate buying about 10 percent more decking material than needed. This gives you some room for errors. Multiply that number times the cost of a single board. If you want packaged material, figure how many packages you need to complete your deck. Multiply that number times the cost of a single package. If you intend to replace railings, stairs and support beams, add those costs into your estimate. Don’t forget to include the cost of brackets, flashing and screws. Home Depot suggests using “5 pounds of screws (or nails) for every 100 square feet of decking.” Add in the cost of any accessories you need. These may include lights, builtin seating, storage or planter boxes.

Determine how much cement you need for the footings or pads. If the footings are round, take half of the holes' diameter and square it, or multiply it times itself. Multiply that number times 3.14. Take this number and multiply it times the holes’ depth. Divide the answer by 1,728. This tells you how much concrete it will take for one hole. If the footing or pad is rectangular, measure the length and width. Multiply those two numbers. Multiple the resulting number by the depth of the holes. Divide this answer by 1,728. This tells you how much concrete it will take for one rectangular footing or pad. Multiply the final number by the number of footings you need. This give you the full amount of concrete needed for the job. Determine how many bags of concrete you’ll need to purchase, and multiply that number by the price of a single bag.

Call three different contractors if you intend to hire someone. Get full bids from each. Even if you choose to do it yourself, compare your material list to theirs to ensure you didn’t leave anything out.
Tips & Warnings
 If you use natural wood, add in the cost of stain or water repellent.
 Your local home improvement store may have an "estimate calculator" for you to use.
 If you plan to do the work yourself but don't have all the tools, include the cost to purchase or rent tools.
 Always expect the project to cost more than your estimate.