The best way to get new outdoor cushions that perfectly fit your patio furniture is to measure your old cushions before throwing them away. To ensure a good fit, try to match the shape, thickness, width, and length of each cushion you replace.
If you no longer have the old outdoor cushions, or your patio furniture never included any, you can measure the furniture itself. With a tape measure, take precise measurements of every seat you want to outfit with a new cushion.
While measuring, pay special attention to the shape of each seat to make sure the new cushions fit snugly. If your seats have curved backs, then a cushion with a square back won’t fit very well. Luckily, outdoor cushions are soft, so even if you can’t find an exact match, something that is approximately the right shape might still work.