A drought in Timberborn stops the rivers, and whatever water your beavers drink after that has to already be sitting in a tank. Enter your population and how long you want to survive, and this tool gives you the stockpile, the tank count, and whether your pumps can refill it all in time.
How long do droughts last?
The game mode sets the range. With the Version 1 presets on the wiki, Easy droughts last 2 to 4 days, Normal 5 to 9 days and Hard 15 to 30 days. The wet (temperate) season between them lasts 16 to 19 days on Easy, 13 to 17 on Normal and only 5 to 8 on Hard. Early cycles are also shorter than the maximum, because each preset starts with a handicap that ramps up over the first few cycles. Plan for the top of the range, not the drought you just had: the button above fills in the longest drought for the selected difficulty.
How the stockpile is worked out
Water to store = beavers x 2.12 x (drought days + buffer days). Food uses the same formula with 2.67 per beaver. On Easy both rates are multiplied by 0.4. The buffer covers the gap between the drought ending and your pumps actually having water to pull again. Two days is our starting point; raise it if your pumps sit in a shallow stream that takes a while to fill.
Example: 50 beavers on Normal facing a 9-day drought with no buffer need about 955 water. That is four Medium Tanks (300 each) or one Large Tank (1,200). The same colony on Hard facing a 30-day drought needs about 3,182 water, which is three Large Tanks.
Which tank to build
The Small Tank holds 30 and costs only 15 logs, so it is the early-game option. The Medium Tank holds 300 for 30 planks and 20 gears on a 2x2 footprint, ten times the water on four times the ground. The Large Tank holds 1,200 on a 3x3 footprint but needs metal blocks. Per tile of ground, the Large Tank stores the most.
Refill check
Tanks only help if they are full when the drought starts. The refill table subtracts what your beavers drink each day from what your pumps make, and divides the stockpile by that surplus. If the answer is longer than the shortest wet season for your difficulty, you will start some droughts with half-empty tanks, and the tool tells you how many more pumps close the gap.
Worked example: 120 Folktails on Normal
120 beavers drink 120 x 2.12 = 255 water a day. A 9-day Normal drought plus a 2-day buffer needs 11 x 255 = 2,800 water: three 
Large Tanks (1,200 each) or ten 
Medium Tanks (300 each). Food for the same 11 days is 120 x 2.67 x 11 = 3,520 items, so plan warehouse space too. To refill 2,800 water inside the shortest 13-day wet season while still drinking, the pumps need 2,800 / 13 + 255 = 470 water a day: ten 
Water Pumps (48.5 each) or two 
Large Water Pumps (242 each).
Drought questions
How long is the longest drought in Timberborn?
With the Version 1 presets, up to 4 days on Easy, 9 on Normal and 30 on Hard. Custom game settings can change this.
How much water should I store for a drought?
Beavers x 2.12 x (drought days + a buffer). For 50 beavers and a 9-day drought that is about 955 water before any buffer.
Is a Large Tank better than Medium Tanks?
Per tile of ground, yes: 1,200 water on 9 tiles against 300 on 4. It costs 80 planks, 60 gears and 30 metal blocks, so most colonies build Medium Tanks first.
Do crops survive a drought?
Not for long without water. Fields need irrigated ground to keep growing, so store food as well as water, or keep a reservoir that irrigates your farms.