What the result means
The headline range is the stretch of days during which the pot will most likely reach its drying rule. Do not water on the first day of it: start testing the day before, as the line under the range says, and water when the test says so. If the mix is still damp at the end of the range, your spot is cooler or darker than the inputs suggest, and next time you can choose one step lower in the light list. The volume line assumes you pour until water drains from the hole, which Clemson HGIC recommends because it carries away excess salts and wets the lower two-thirds of the pot, where most roots live. Empty the saucer afterward.
The weight test is the one experienced growers lean on. Lift the pot right after a thorough watering and remember the heft. The calculator estimates how many ounces of water a pot that size holds and how much of it should be gone before the next drink, given the plant's rule. A hollow sound when you knock on a clay pot points the same way, Clemson notes, because drying mix shrinks away from the wall.
The drying rules behind each plant
Every plant guide on this site stores one rule, read from its sources. Each rule maps to a reference range for a 6-inch plastic pot with a hole, standard potting mix, bright indirect light, summer and average humidity:
Those reference ranges are our model, not a published figure, and the methodology page says so. They exist so the multipliers have something to scale.
How each factor moves the range
Clemson HGIC lists why two identical plants drink differently: porous clay loses water through its walls and needs watering more often than plastic or glazed pots; a large plant in a small pot dries faster than a small plant in a large pot; peat-heavy mixes hold more moisture than loose blends of bark, sand and perlite; and a plant in a warm, dry, sunny spot needs water more often than one in a cool, dim room. Plants also use less water while resting. The calculator turns each of those statements into a multiplier: unglazed terracotta ×0.75, low light ×1.45, winter ×1.6, dry air ×0.85, orchid bark ×0.7, no drainage hole ×1.4. Open “How this range was built” under the result to see the product for your own inputs.
What it cannot know
It cannot see the root ball. A plant that has filled its pot with roots dries out far faster than the model expects, and a recently repotted one far slower. It assumes a standard pot about as tall as it is wide, so a shallow bonsai dish or a tall cylinder will behave differently. It ignores heating vents, grow lights and drafts. Those are the cases where the finger test matters most, and where the diagnosis tool helps if the leaves start telling you something went wrong.