Two numbers, and only the second one is work
The count falls out immediately. Sixty feet of pier at eight foot maximum spacing is 7.5 bays, which rounds to eight, and eight bays means nine bents if there is one at the shore end. At two piles a bent that is eighteen piles. The actual spacing is then 60 divided by 8, or 7.5 feet, not the eight you started with — rounding the bay count up always tightens the spacing, never loosens it, which is the direction you want.
The lengths are where the time goes. Every pile is embedment, plus the water depth where it stands, plus the height from the water to the underside of the cap, plus whatever you are ordering for cut-off. Three of those four are the same at every bent. The water depth is not, and on a bottom falling from one foot at the shore to six at the outer end, the outer piles are five feet longer than the inner ones. That is a whole step in most supply, so a pier ordered off one length either wastes five feet on half the piles or comes up short on the other half.
The defaults, bent by bent
Eight feet of embedment, three feet of deck above the water level the depths were taken at, twelve inches of cap and decking, two feet of cut-off. The pile top has to finish three feet less twelve inches, so two feet above that water level. The fixed part of every pile is therefore eight plus two plus two, which is twelve feet, and the only thing that varies is the depth.
At the shore bent the water is a foot deep, so the pile needs thirteen feet and orders as fifteen in five foot steps. At the outer bent the water is six feet, the pile needs eighteen, and it orders as twenty. Nine bents at two piles each with lengths climbing from fifteen to twenty is a schedule, not a quantity, and it is the schedule the yard needs.
Straight lines and real bottoms
Depths between the ends are interpolated linearly here, which is a convenience and not a claim. Lakes and rivers put ledges, holes, old cribwork, soft pockets and bars between one end of a pier and the other, and any of them moves a real pile several feet off the schedule. Sounding at every bent position takes an afternoon with a marked pole and it is the difference between a delivery that works and one that has two piles standing proud and two that will not reach.
It also matters which water level the depths were taken at. Every vertical number on this page is measured from that surface, so a set of soundings taken at summer pool and a deck height taken from a drawing referenced to winter drawdown will not add up, and the error goes into every pile at once.
What is deliberately not here
No embedment figure. How far a pile has to go into the bottom is a function of what the bottom is made of, what the pier has to resist, and how the pile is installed, and none of that is arithmetic. It comes from the engineer and from the contractor who is driving or jetting them, and this page takes it as an input for exactly that reason.
No spacing figure either, no pile diameter, and no view on bearing, uplift, ice, scour or lateral load. The page counts and adds. Everything it adds up is a number that came from somewhere else.
Anything built in, on or over water is permitted work in most places, and the permission usually comes from more than one desk. The state environmental or natural resources agency, the Army Corps of Engineers district for the waterway, the local building department, and on many lakes the utility or authority that holds the shoreline licence all have a say, and what each one wants is different from one state and one waterbody to the next. None of it is on this page. Start with the state agency and the Corps district for the water you are on, and get a marine contractor who works that lake or that stretch of river involved before the arithmetic turns into an order.
Questions people ask
How far apart should dock pilings be?
That spacing is a design output, not a rule of thumb, and this page takes it as an input rather than supplying one. What sets it is the span of the stringers you are using at the loading the pier is designed for, together with everything the piles have to resist sideways, and it is decided by whoever designs the structure or by the span table your project is held to. What the arithmetic can tell you is what a chosen spacing does to the count: a sixty foot pier at eight foot maximum spacing comes out at eight bays and nine bents, and the actual spacing lands at 7.5 feet because rounding the bay count up always closes the spacing rather than opening it.
How long do my pilings need to be?
Add four things, three of which you already know. Embedment below the bottom, the depth of water where that pile stands, the height from the water level you measured at to the underside of the cap, and the cut-off you want to be able to trim. On the defaults here that is 8 plus the depth plus 2 plus 2, so a pile in one foot of water needs thirteen feet and one in six feet of water needs eighteen. The embedment is the term nobody can look up: it depends on the bottom material and on the installation method, and it comes from the engineer and the pile contractor.
Why are the pilings all different lengths?
Because the bottom falls away and the deck does not. The deck is level by definition, the embedment is normally the same at every bent, and the water depth is the only term that changes — so the pile length changes with it, foot for foot. On a bottom going from one foot at the shore to six at the outer end, the outer piles are five feet longer, which is a full step in most supply. Ordering a pier off one length means either paying to cut five feet off half of them or discovering that the outer bent does not reach.
How much extra length should I order for cut-off?
That is a judgement about your installation rather than a calculation, which is why it is a field here. Two feet is a figure people commonly work to, and what it buys is the ability to trim every top to one line after driving, plus some insurance against a pile that meets refusal earlier than expected. Driving is not precise: piles wander, they stop short on something buried, and they go deeper than planned in a soft pocket. The allowance is what lets that be a trimming job rather than a re-order, and the contractor who is driving them will have their own view on what it should be.
Can this tell me whether my pier design will hold?
No. It counts bents, multiplies for piles, and adds four lengths together. It does not size a pile, choose a spacing, work out embedment, or have any view on bearing, uplift, scour, ice or lateral load, and it does not know what the bottom is made of. Fixed piers over water are also permitted work in most places, and the state environmental agency, the Corps district for that waterway, the local building department and often a lake authority each have requirements that differ by state and by waterbody. Get the design from an engineer and the permitting from those bodies before the schedule above becomes an order.