The bunk is an edge
Everything on this page comes out of one multiplication: head count times inches per head. Forty head at 24 inches is 960 inches, which is 80 feet of bunk face. That is the number. What happens after it is division — by two if they can eat from both sides, by whatever fraction of the group actually has to be at the bunk at one moment.
With the defaults, 40 head each with their own space at 24 inches gives 80 feet of edge. Eaten from one side, with six inches lost at each end, that is a run of 81 feet, which comes out as nine 10 ft sections and 90 feet built. The usable face is 89 feet, which at 24 inches is 44 spaces — four more than the group needs, because sections do not come in fractions.
The second side halves the bunk and does not touch the concrete
Switch the same 40 head to eating from both sides and the run drops from 81 feet to 41. Half the structure, half the posts, half the length to fill. The apron does not follow it. Apron area is run times width times sides, and halving the run while doubling the sides cancels exactly — 81 feet one-sided at 10 feet wide is 810 square feet, and 41 feet two-sided at 10 feet wide is 820. Ten square feet apart, and the ten is not a rounding artefact: it is the end allowance being counted twice instead of once, because a two-sided bunk still has two ends but now needs the allowance on both faces.
That is worth knowing before the concrete is ordered, because the intuition runs the other way. People expect the two-sided layout to be cheaper across the board and it is not. It is cheaper in bunk and no cheaper at all in slab, and on a feeding apron the slab is usually the larger of the two numbers.
Turns at the bunk change everything and are not this page to decide
Set the group to two head sharing a space and the 80 feet becomes 40. That halves the bunk, halves the apron and halves the cost, and whether it is a reasonable thing to do depends entirely on what you are feeding, how it is fed, how often, and how much competition there is between the animals in that group — none of which a calculator can see. The field exists so that the arithmetic can follow your decision. It is not the decision.
Headlocks do not always agree with your figure
Headlock units come at a spacing set by whoever made them, and it is rarely the same as the inches per head you work to. Put a spacing in and the page prints both counts side by side and says which one is setting the number. If the locks are wider than your figure, the locks win and the group gets fewer places than the length suggests. If they are tighter, your figure wins and the extra locks are just extra locks.
What the length does not tell you
A bunk that is the right length and the wrong height, or the right length with feed only reaching the first thirty feet of it, is a bunk with a problem the arithmetic cannot see. So is one that cannot be cleaned out, or one where the animals at one end are being kept off by the animals at the other. Length is the part that is easy to compute and it is genuinely worth computing, but it is one input into a design rather than the design.
Questions people ask
How many feet of feed bunk do I need per head?
That comes out of the inches per head you work to, which is a field on this form rather than a number this page holds. Put your own figure in — from your extension or advisory service, your vet, or whoever you take guidance from — and the page multiplies it by the head count and divides by the number of sides. At the defaults on the form, 40 head at 24 inches each is 80 feet of bunk face, or 41 feet of run if both sides are used.
Does a two-sided bunk save concrete?
No. It saves bunk. Apron area is the run times the apron width times the number of sides, so halving the run and doubling the sides cancel out. At the defaults the one-sided layout takes 810 square feet of apron and the two-sided takes 820 — the difference is the end allowance being needed on two faces instead of one. The saving from going two-sided is entirely in the structure.
Why does the page build more sections than the run needs?
Because sections come whole. A run of 81 feet made of 10 ft sections is nine sections and 90 feet built, and the extra nine feet gives four more feeding spaces than the group needs. The page prints the built length and the spaces it actually gives so the overshoot is visible rather than buried.
What is the end allowance for?
The post, the end cap, the corner an animal cannot get its head into. Every bunk loses some face at each end and the amount depends on how yours is built, so it is a field. Set it to zero and the arithmetic runs on the bare length. It is also the only reason the one-sided and two-sided apron figures differ at all.
Can this tell me whether my bunk is long enough?
No, and it does not try. It takes the inches per head you entered and reports how many spaces a given length gives at that figure. Whether that figure is right for your animals, your feed and your management is a veterinary and advisory question, and whether the result meets any rule that applies to you is a question for whoever regulates the operation where you are.