Area gets you the surface. What it does not get you is the frame underneath, the edge treatment around it, or the base beneath a patio, and those are usually the larger part of the order.
Length times width gives the surface. Take it at the outside edge of the finished structure, and split L-shapes or wrap-around decks into rectangles rather than trying to handle them as one.
Then take three more measurements that the area does not contain.
The perimeter, because the edge is treated differently everywhere. On a deck it is fascia board and possibly railing. On a patio it is edge restraint, which holds the whole thing together and without which pavers migrate outward over a few seasons.
The height above ground, for a raised deck. This decides post lengths, whether railing is required by code, and whether steps are needed. Measure at several points, because the ground below is rarely level.
The fall, for a patio. A patio is laid with a slight slope away from the building so water drains rather than pooling against the wall. That means the finished level at the far edge is lower than at the house, and you need the height difference to work out the base depth at each end.
Board direction is a decision that changes the material count, so it belongs in the measuring stage.
Boards run perpendicular to the joists, so the direction you want the boards to run dictates which way the frame runs underneath. The number of boards is the deck dimension across the board direction, divided by the board coverage width including the gap between boards.
That gap matters. Decking is laid with a spacing for drainage and movement, commonly around an eighth of an inch, and across a wide deck those gaps add up to a meaningful distance. Calculating on the bare board width rather than board plus gap over-orders slightly, which is a safe direction to be wrong in.
Board length against deck length is the more consequential question. If your deck is 14 feet across and boards come in 16 foot lengths, every board wastes two feet unless you can use the offcut elsewhere. Checking available board lengths against your dimensions before finalising the deck size can save a surprising amount.
Joists at a set spacing across the deck length, plus beams, plus posts, plus the ledger against the house. It is a different measurement from the surface area and it is usually more material than people expect. Joist spacing is set by span tables rather than preference, so it is a specification question rather than a measuring one.
A patio is a stack. Paving on a bedding layer on a compacted sub-base on excavated ground.
So the same area gets calculated at several depths, and the total excavation depth is the sum of all the layers plus the paving thickness. Getting this wrong means either digging twice or finishing with a patio that sits proud of the door threshold.
Each layer's volume is area times depth, divided by 324 to give cubic yards. The conversion is here. Specific depths depend on ground conditions and expected load, so take those from your supplier or the paving manufacturer rather than a generic figure.
The excavated material has to go somewhere too, and it is more than people expect. A 200 square foot patio dug to 8 inches produces around five cubic yards of spoil, which is several skip loads.
Rectangular decks and patios waste very little. Curves, angled edges and anything that has to fit around an existing feature waste considerably more, because every edge unit is cut and the offcut rarely fits elsewhere.
Diagonal or herringbone paving patterns waste more again for the same reason a diagonal floor does: every perimeter piece is a cut. If you are laying at 45 degrees, allow accordingly.
Order slightly long on paving in particular, because it is batch-manufactured and colour varies between production runs. Driveways follow similar logic at larger scale, and measuring the plot is covered here.
You will find our overall picks on the front page, with the reasoning behind each one.