Blending combines two real batches to land at a gravity in between them — useful for correcting a batch that came out at the wrong strength, evening out a small brew-day inconsistency, or deliberately creating a new blend from two finished beers.
A worked example
15 L of a stout reading 1.014 blended with a stronger batch reading 1.030, targeting a 1.020 result, needs 9.0 L of the stronger batch, producing 24 L total at the target gravity. Reproduce this exactly — swap in your own two batches' actual gravities and volumes.
The algebra behind it
Blending works in gravity points, the same additive unit dilution uses. With batch A's points, batch B's points, and a target points value, the volume of B needed to hit that target given a fixed volume of A is:
volumeBNeeded = volumeA x (pointsA - targetPoints) / (targetPoints - pointsB)
This only produces a sensible, positive answer when your target gravity actually sits between the two batches' gravities — you can't blend two batches to land outside the range either one alone occupies, which is the one hard constraint on this technique. If you enter a target above both batches or below both, the calculator will tell you rather than return a nonsense number.
Practical considerations beyond the gravity math
Blending mixes more than gravity — colour, bitterness, and flavour character all combine too, in proportion to the volumes actually mixed, not in any way the gravity-points formula accounts for. Two batches blended to a perfect target gravity can still taste unbalanced if their flavour profiles clash rather than complement each other — a very hoppy batch blended with a malty one, for instance, produces a genuinely different beer than either component, not just a strength adjustment. Taste both batches individually before committing to a blend ratio, and consider blending a small test volume first (a few hundred millilitres of each, mixed at your calculated ratio) to check the flavour result before committing whole batches.
When blending beats dilution
If your goal is adjusting gravity without proportionally diluting colour and bitterness the way plain water would, blending with a second batch is the tool that gives you that independent control — see dilution vs. blending: when to use which for the fuller comparison, and my OG came in too low — how do I fix it for a specific case where blending mechanics (using a concentrated sugar solution as "batch B") fix a too-weak batch rather than requiring a genuine second full batch.