Both tools solve "my gravity is wrong, fix it" — the difference is what else moves along with the gravity when you do.
Dilution: one batch, plain water, gravity moves alone
Diluting adds water to a single batch and lowers its gravity, and only its gravity — colour, bitterness, and everything else already dissolved in the wort or beer gets spread across a larger volume proportionally, but nothing new is introduced to change their character. 19 L reading 1.070 diluted down to a 1.060 target needs 3.17 L of water, landing at 22.17 L total. Run this exact example. Use dilution when your wort or beer is simply stronger than intended across the board and you're happy for colour and bitterness to come down proportionally along with gravity — a too-concentrated boil from short pre-boil volume is the classic case; see how to dilute a too-strong wort.
Blending: two real batches, everything moves independently
Blending combines two actual batches — different gravity, different colour, different bitterness, different flavour — to land at a gravity in between the two. Because you're mixing in a second batch's full character rather than plain water, colour and bitterness don't move in lockstep with gravity the way they do with dilution; a strong, dark batch blended down with a weaker, pale one moves all three properties by different amounts depending on how different the two batches actually are. A worked example: 15 L of a 1.014 stout blended with a stronger 1.030 batch to hit a 1.020 target needs 9.0 L of the stronger batch, for 24 L total. Run this example. See how to blend two batches of beer for the full mechanics.
The actual decision
- You only have one batch, and it's simply too strong: dilute. There's nothing to blend with, and water is the only tool that only touches gravity.
- You have two batches and want a specific gravity in between, without dropping colour and bitterness proportionally: blend. A batch that came out too pale can be darkened by blending with a small amount of something much darker, in a way plain water can never achieve.
- Your OG came in too low, not too high: neither tool as described here fixes that directly — plain water only ever lowers gravity further. See my OG came in too low — how do I fix it, which uses the blending tool's mechanics but with a concentrated sugar solution playing the role of "batch B" instead of a second full batch.
- You're trying to raise ABV in an already-fermented beer: dilution and blending both work in gravity points, which describe fermentable extract, not alcohol directly — see how does topping up change my ABV for what changes once alcohol, not just sugar, is already part of the liquid.
A caution that applies to both
Neither tool undoes itself. Add too much water and there's no clean way to remove it again short of blending in something stronger; blend in too much of a second batch and you've permanently combined them. Measure your actual gravity and volume carefully before committing, and when you're not fully certain, add in smaller increments and re-measure rather than dosing the full calculated amount in one go.