Subtract downward from the one thing that is level
A deck has exactly one reliably flat reference and it is the finished surface. Everything else, including the ground, is a variable. So the way to get post lengths right is to start at the deck surface, subtract each layer of the assembly on the way down, and arrive at the top of the pier.
The layers are the decking, the joist, the beam if the joists sit on top of it, and however far the pier or post base stands above the ground. With 5/4 decking, 2x10 joists, a beam of the same depth and six inches of pier showing, that is 1 plus 9.25 plus 9.25 plus 6, which is 25.5 inches of assembly. Subtract that from a 96 inch deck height and the post at that point is 70.5 inches, which is 5 feet 10-1/2 inches.
If the joists frame into the beam on hangers rather than sitting on top of it, the beam depth comes out of the subtraction entirely, because the top of the beam is now at the same level as the top of the joists. Set the beam depth field to zero for that case. It is a nine inch difference in post length, which is enough to matter.
Then the ground, which is the part people guess at
Ground that looks flat routinely falls three to six inches across a normal deck footprint. The eye is very bad at this because there is no reference to compare against, and a lawn with any texture at all hides the fall completely. The consequence of guessing is that you cut a set of identical posts, stand them up, and find that the beam only touches three of them.
Take a real reading at every post. A string line pulled level between two stakes, a long straightedge with a level on it, a water level for long runs or a laser if you have one. Pick one post as the reference, call it zero, and record how far the ground at each other post is below that. Negative numbers are fine and mean the ground is higher there.
Then decide what stays constant. There are two honest answers and the calculator supports both:
| Approach | What varies | Suits |
|---|---|---|
| Each pier held the same height above its own grade | The posts, one length each | Most sites. Piers look consistent, posts get cut to fit. |
| All pier tops poured to one elevation | How far each pier stands proud of the ground | Modest fall. All posts identical, one cut length. |
The second is tempting because a single post length is easy, and it works until the fall gets large enough that the pier at the low end is standing a foot and a half out of the ground looking like a monument. Somewhere around eight to twelve inches of fall most people switch to the first approach.
Which point you measured from, and why to write it down
Every number on the cut list is relative to one post, and that post is whichever one you happened to start at. If somebody re-measures next week from a different corner, all the drops change sign or magnitude and the cut list is silently wrong. Mark the reference post on the drawing, on a stake, and on the cut list itself.
The same applies to what the deck height means. Ninety-six inches to the top of the decking is not the same as ninety-six inches to the top of the joists, and the difference is a full board thickness that will show up as a step at the door. If the deck meets a door threshold, work the whole calculation backwards from the threshold height instead, because that is the dimension that has to be right.
Grade also moves. Ground gets landscaped, mulch gets added, soil settles around a fresh excavation. A pier held six inches above grade today is held four inches above grade after two years of mulch. That is an argument for the generous end of the standoff range rather than the tight one.
Cut long, trim in place
String line readings are good to perhaps a quarter of an inch on a decent day. The beam sitting on the posts is good to whatever the beam is. So if the sequence allows, stand the posts long, set the beam, mark the posts against the beam in place, and cut them there. A post cut a quarter inch short is firewood and a delay; a post cut two inches long costs one crosscut.
Where that is not practical, because the posts have to be fixed to their bases before anything else can happen, cut to the list and expect to shim. Which is fine at the top of a post, where a shim is under compression and captured, and not fine at the bottom, where a shim under a post base is a hinge.
The rest of the deck is the usual sequence. Footing loads and how much each post is carrying come from the deck footing load calculator, the concrete for the piers from the concrete footing calculator, the frame above from the deck framing calculator, and the ledger, which is the connection that matters most and is worth reading about properly, from the deck ledger fastener calculator. If the fall under the deck turns out to be significant, the deck skirting and lattice calculator deals with closing it in without trapping the damp, and the slope and grade calculator converts the fall you measured into a percentage or an angle if you need it in those terms.
Questions people ask
How do I measure the grade drop at each post accurately?
Pick one post as the reference, drive a stake, and get a level line from it. A string line with a line level works to about a quarter inch over a short run and gets worse with sag over a long one. A long straightedge with a spirit level on it, leapfrogged along, is slower but more reliable. A water level handles long runs and corners without line of sight. A rotary laser is the easy answer if you have access to one. What does not work is eyeballing it or trusting the look of the lawn, because a lawn hides several inches of fall without any visual cue at all.
Should the pier tops all be at the same level or the same height above ground?
Both are done and the choice is about how it looks and how much fall there is. Holding each pier the same height above its own local grade keeps the piers looking consistent and pushes all the variation into the post lengths, which nobody sees once the deck is on. Pouring all the pier tops to one elevation gives you identical posts, which is convenient, but on a sloping site the low end pier ends up standing well out of the ground. Under about eight inches of fall the level option is usually fine. Beyond that most people go the other way.
Do I subtract the beam depth or not?
It depends on how the joists meet the beam. If the joists sit on top of the beam, the beam depth is a full layer between the joists and the post, so subtract it. If the joists frame into the side of the beam on hangers, the tops of both are at the same level and the beam contributes nothing to the stack, so set the beam depth to zero. Getting this wrong is a nine to eleven inch error in every post, which is enough that you notice at once, but it is also the single most common mistake in this calculation.
How far above the ground should the bottom of the post be?
Above it, in any case. A post bearing directly on soil, or on a pier flush with the soil, sits in the wettest place available and holds water against its end grain. Standing the pier proud of the ground gives the post base somewhere to drain and keeps splash off the timber. Six inches is a common figure to work from and some jurisdictions specify a minimum. Remember that grade rises over time as mulch and soil accumulate, so a standoff that is exactly adequate on the day of the pour will not be in five years.
Can I use the offcuts?
Sometimes, and it is worth knowing before you order rather than after. Buying a set of 10 foot pieces to make 5 foot 10 inch posts leaves just over 4 feet each time, which is too short for another post and too good to burn. Those offcuts are useful as short posts on the low side of a stair landing, as blocking, or as stakes. If the offcut is going to be substantial across a lot of posts, check whether a shorter stock length covers the longest post, since the price per foot on shorter stock is often better and the waste is much less.