Span is squared, tension is not
The whole page comes off one line: sag equals the weight of a foot of wire, times the span squared, divided by eight times the tension. Span appears squared and tension appears once. That asymmetry is the practical content, and it is why a post buys more than a harder pull.
With the defaults — 16 feet between posts, wire at 92 lb per thousand feet, 250 lb on the strainer — the sag is 0.14 inches. Put a post in the middle and it becomes 0.04 inches, a quarter. Double the tension instead, to 500 lb, and it becomes 0.07, a half. The post is twice as effective and it does not add anything to what the end assembly has to hold, while doubling the tension doubles that directly.
What the line posts are actually for
Take every line post out and leave the same wire at the same tension spanning brace to brace — 660 feet on the defaults — and the sag is 20.04 feet. That is a ratio of 1,702 to one against the sag between posts, and it comes entirely from the square: 660 over 16 is 41.25, and 41.25 squared is 1,701.6.
It is worth being clear about what that means. A line post on a wire fence carries almost nothing downward. The weight of a whole span of all five wires, on the defaults, is 7.36 pounds. The post is not holding the wire up in any load-bearing sense; it is shortening the span, and shortening the span is squared in its effect. That is a different job from the one the end assembly does, and it is why line posts can be light and ends cannot.
The temperature part is the one that surprises people
Warm the wire 60 degrees above the day it was tensioned and, at 6.5 millionths per degree, it wants to be 0.00039 of its length longer. Over a 16 foot span that is six thousandths of a foot — 0.0749 of an inch. Nothing you could find with a tape.
Except that sag is a square root of that slack, not a proportion of it. The cold sag was only using 0.00014 percent of the span in extra length. The warm wire has 0.039 percent to play with — 271 times as much slack — and sag follows the square root of it, so it goes up by 16.5 times. The sag runs from 0.14 inches to 2.33, and the tension that would hold the wire at that sag is 15 pounds instead of 250.
That number is an upper bound and the page says so on the result. Steel wire is stiff, so in a real fence most of a temperature swing turns into a change in tension rather than into visible sag, and the answer sits somewhere between the two figures. Working out where needs the modulus and the cross-section of your specific wire. What the bound does establish is why a high-tensile fence has springs and ratchets in the line at all: the slack that has to be managed over a season is tiny in length and enormous in its effect on tension.
What the ends carry
Five wires at 250 pounds is 1,250 pounds pulling straight along the fence line at each brace, all day, in both directions, for as long as the fence stands. At a corner where two lines meet at a right angle it goes up by the square root of two, to about 1,768 pounds on the diagonal. That is the number that decides how an end assembly is built, and it has nothing to do with sag — it is why a fence built with good line posts and a poor brace starts leaning in its first season.
What is not in here
No breaking load, no working load, no safety factor, no post capacity, no brace design. This page will happily compute the sag for a tension your wire cannot take, because it has no idea what your wire can take and no business guessing. It also treats the wire as a parabola with self-weight only, which is close enough for a fence span and wrong the moment anything hangs on it, leans on it or lands on it.
Questions people ask
How much does a fence wire sag between posts?
Sag is the weight of a foot of wire times the span squared, divided by eight times the tension. At the defaults on the form — 16 ft spacing, 92 lb per thousand feet, 250 lb of tension — that is 0.14 inches, which is why a properly strained wire looks dead straight. Over the full 660 ft between braces with no line posts under it, the same wire at the same tension sags 20.04 feet — 1,702 times as much, because 660 over 16 is 41.25 and 41.25 squared is 1,701.6.
Is it better to add a post or pull the wire tighter?
Adding a post, by a factor of two, and it is free at the ends. Halving the spacing quarters the sag because span is squared in the formula. Doubling the tension only halves the sag, and it doubles the pull the end assembly has to hold — 1,250 pounds becomes 2,500 on a five-wire fence. The page prints both so the comparison is in front of you.
Why does my wire go slack in hot weather?
Because sag responds to the square root of the slack. A 60 degree rise at 6.5 millionths per degree stretches the wire by 0.00039 of its length — six thousandths of a foot, 0.0749 inches, over a 16 ft span — which is 271 times the slack the cold sag was using, and that alone is enough to take the sag from 0.14 inches to 2.33 and the tension from 250 pounds to 15. The page gives that as an upper bound: real steel is stiff and takes most of the swing as tension change rather than visible sag, which is exactly what in-line springs are there to absorb.
How much pull does a fence end assembly take?
The tension per wire times the number of wires, pulling horizontally along the line. Five wires at 250 pounds is 1,250 pounds at each brace. A corner where two lines meet at a right angle sees up to 1,768 pounds on the diagonal. This page reports that load and states nothing about whether any particular brace, post or anchor can hold it.
Does this tell me what tension to use?
No. Tension is a field you fill in, from your own gauge and your own decision, and the page holds no rated strength, working load or safety factor for any wire. It will compute the sag for a tension that would part the wire, because it does not know what your wire is. Those figures come from the wire supplier.