The leach fraction is a divisor
This is the arithmetic that catches almost everyone. If you want 20 percent of what you pour to run out of the bottom, you do not pour 20 percent extra. The runoff is a share of the total poured, not of the amount kept, so the total is the retained volume divided by 0.8 — a multiplier of 1.25.
The gap grows fast. At a 20 percent leach fraction the multiplier is 1.25. At 33 percent it is 1.49. At 50 percent it is 2.0, meaning half of everything you pour is destined for the saucer. Treating the fraction as an addition understates the pour at every level, and the error is exactly what makes people conclude the pot never fully wets.
Two numbers you have to measure yourself
The whole calculation rests on the water your mix holds when it is full, expressed as a share of its volume, and on where it currently sits between empty and that point. Neither can be looked up honestly, because both depend on the specific blend in the specific pot with the roots that are actually in it.
The first one is a weighing exercise and it only has to be done once per mix. Take the pot at its driest, put it on a scale, note the reading. Water it until it stops dripping and weigh it again. The difference in pounds divided by 8.345 is the gallons it took, and dividing that by the mix volume in gallons gives the percentage this page asks for. At the defaults, 4 quarts of mix at 45 percent holds 57.6 fluid ounces, which is 3.75 pounds of water.
The second one is the same scale used as a gauge. Once you know the dry weight and the full weight, any reading between them locates the pot on the scale as a percentage, and it does it far more reliably than a finger or a meter.
Working the default numbers
Four quarts of mix, 45 percent water holding, sitting at 25 percent and going back to 100, with a 20 percent leach fraction. Capacity is 57.6 fluid ounces. It currently holds 14.4, and the target is 57.6, so it keeps 43.2. Divide by 0.8 and the pour is 54.0 fluid ounces, of which 10.8 goes out the bottom.
That runoff figure matters, because 10.8 fluid ounces is more than a common 8 ounce saucer holds. The page flags the overflow and says what size saucer would take it. This is the sort of thing that only becomes obvious once the water is on the carpet.
The weight check is the last line. Keeping 43.2 fluid ounces means the pot gets 2.82 pounds heavier, which is a large enough change that any kitchen scale sees it clearly.
Where the model stops
This is a volume balance and nothing more. It assumes the water distributes through the mix rather than channelling down one side and straight out, which is exactly what a mix that has dried hard will do — in that case the runoff appears immediately, the leach fraction looks enormous, and almost nothing has been retained. If the pour and the weight change do not agree, that is usually why, and the disagreement is more informative than either number alone.
It also says nothing about where the runoff goes chemically. Water carries dissolved material out with it, in an amount that depends on your supply and on anything you have added, and this page neither models nor advises on that.
Hazards worth naming
Water standing in a saucer is a small stagnant pool indoors; for most households it is unremarkable, and for anyone whose immune system is compromised it is worth raising with a doctor. Potting media itself carries organisms for the same reason. Water and mains electricity near each other need care, particularly around a fixture that has been on for hours and is hot. And a large pot at full water weight is a genuine load for a windowsill, a plant stand or a wall shelf.
Questions people ask
Why does the page not tell me how often to water?
Because it cannot, and neither can anything else that does not know the plant, the pot, the room, the light and the season. Interval depends on how fast the pot loses water, which changes week to week. What this page does is answer the question that is actually arithmetic: given where the pot is now and where you want it, how much water is that, how much runs out, and what should the scale read afterwards. When to do it is a question for whoever grows the plant successfully, a supplier, or a local extension service.
What is a leach fraction and why would I want one at all?
It is simply the share of the water you pour that leaves through the drainage holes rather than staying in the mix. Setting it to zero on this page is legitimate and the arithmetic handles it: the pour equals the retained volume and there is no runoff. Whether you want runoff, and how much, depends on your water supply and on what you add to it, and that is a growing question rather than a calculation. The page takes your figure and does the volume arithmetic on it either way.
The pot weighs much less than the calculated gain suggests. What went wrong?
Almost always channelling. Mix that has dried hard develops paths where water runs straight through without wetting anything, so the runoff appears within seconds and the weight barely moves. The tell is the timing: retained water takes a while to show up as runoff, and channelled water is out the bottom almost immediately. The calculation assumes the water reaches the mix, and when it does not, the scale is the honest instrument and the volume is the misleading one.
Can I use this for a pot with no drainage hole?
The retained-volume half works: capacity, current level, target and the weight change are all still valid. The leach fraction half does not, because there is nowhere for runoff to go, so set it to zero. What that means is that anything you pour beyond the capacity stays in the bottom of the container as free water, which the arithmetic will not warn you about because it is a volume calculator and not a judgement. Whether standing water in the base is a problem for a given plant is not something this page addresses.
How do I get the mix volume if I only know the pot size?
Measure the inside of the pot and use the pot size step-up calculator, which handles tapered round pots properly rather than treating them as cylinders, and reports the volume at whatever fill level you actually use. Do not use the nominal size on the pot: trade sizes overstate real volume by a variable margin, and that error goes straight through into every number here. If you want the figure for a batch of identical containers along with bags and loaded weight, the growing media and container volume calculator covers that case.