The shortcut and what it actually gives you
Take the defaults: 22 lb of lean carrying 10 percent of its own fat, back fat at 85 percent, and a 25 percent target. The shortcut is to make the fat trim a quarter of the pile, which is 7.33 lb into the 22, for 29.33 lb of block. Work out what is in it — 2.20 lb of fat from the lean, 6.23 from the trim, 8.43 in total — and the blend is 28.75 percent, not 25.
The right answer is 5.50 lb of fat trim into all 22 lb of lean, giving 27.50 lb of block at exactly 25 percent. The difference is not enormous on the plate but it is nearly two pounds of fat trim, and on a hundred pound batch it is eight.
It does not always overshoot
This is the part worth knowing, because the shortcut is not reliably wrong in one direction. Adding fat trim as a fixed share of the pile lands at the lean stream's fat percentage times the lean share, plus the fat stream's percentage times the target share. With 10 and 85 aiming at 25 that is 28.75, an overshoot. Put 2 percent lean and 60 percent fat trim in aiming at 30 and the same shortcut gives 19.4 percent — an undershoot of just over ten points.
The crossover is when the fat the lean carries, spread over the lean share, equals the lean the fat trim carries, spread over the target share. Nobody is going to hold that in their head, which is the argument for doing the two-line blend properly instead of learning a rule about when the shortcut is safe.
Which stream runs out
At the defaults the lean runs out first: 22 lb of lean only wants 5.50 lb of fat trim, so 6.50 lb of the 12 is left over. That is the usual situation on a home batch, because fat trim accumulates faster than anybody uses it. The page says which way round it is and how much is left, because the answer decides whether you go looking for more shoulder or start rendering.
Casing length is a square law
The stuffed weight at the defaults is 29.70 lb, which at 0.037 lb per cubic inch is 803 cubic inches. Through a 1.25 in casing that is 654 inches, or 54.5 feet, and at 6 inch links it is 109 links weighing 4.36 oz each.
Go to a 1.5 in casing and the same mix only needs 37.9 feet, because the cross section goes as the square of the diameter. That catches people out at the sausage stuffer far more often than the volume does — a quarter inch on the diameter is a third off the footage, and a bundle of casings bought for one size is not a bundle for the other.
What this page will not do
It computes no cure of any kind. There is no field for curing salt, no nitrite figure, no concentration and no time, and that is deliberate rather than an oversight. Fat ratio is a texture decision and safe to work out on a calculator; cure is not, the margin between a working amount and a poisonous one is narrow, and the correct number depends on the product, the method and the mass in ways a general form cannot see. Take that arithmetic to a tested formulation, to your cure supplier for the product in your hand, and to somebody qualified.
Questions people ask
How much fat do I add to make 80/20 sausage?
Less than 20 percent of the pile, and how much less depends on what your lean already carries. With lean at 10 percent fat and back fat at 85, hitting a 20 percent blend takes 0.154 lb of fat trim per pound of lean, so 22 lb of lean takes 3.38 lb — about 13 percent of the finished block rather than 20. Put your own two percentages in the form; the answer moves a lot with them.
Why does adding 25 percent fat not give me 25 percent fat?
Because your lean is not lean. Trimmed shoulder carries around a tenth of its own weight in fat, so that fat is in the block before you add anything. At the default figures the shortcut lands at 28.75 percent instead of 25. With a very lean stream and a soft fat trim it goes the other way and lands under, which is why it is worth doing the two-line arithmetic rather than learning a rule of thumb.
How much casing do I need for a batch of sausage?
It depends on the stuffed diameter far more than on the weight. The default 29.70 lb batch takes 54.5 feet through a 1.25 in casing and only 37.9 feet through a 1.5 in one, because cross-sectional area goes as the square of the diameter. Weigh a stuffed link of your own and measure it once and you will have a pounds-per-foot figure for your own kit.
Does the water count towards the fat percentage?
Not on this page. The fat percentage is worked on the meat block, and the water, ice or other liquid is added afterwards for the stuffed weight and the casing length. That matches how most recipes are written, but not all of them, so check which convention yours uses before scaling anything from it.
Will this calculate my cure?
No, and there is no field for it. Sodium nitrite is toxic at a dose not far above a working one and the correct quantity depends on the product, the method and the mass in ways a general calculator cannot see. Fat ratio is a texture question and safe to work out here. Cure is not, and it belongs to a tested formulation, to your supplier for the specific product you hold, and to somebody qualified in food science.