What This Calculator Measures
Whenever you have two quantities that should stay in a specific proportion to each other — a budget split between two categories, an ingredient ratio in a recipe scaled up, or two portfolio holdings that should stay in a set ratio — drift happens. One side grows or shrinks faster than the other, and the actual ratio moves away from the target. Rebalancing means figuring out exactly how much to add to, or remove from, each side to restore the intended proportion.
This calculator takes your current A and B values, a target ratio (A:B), and lets you choose how the rebalance should happen: adjust both sides to also match a specific total, or hold one side fixed and adjust only the other. It reports the new A and B values plus the exact change (delta) needed for each.
How to Use This Well
- Enter current A and B.
- Set target ratio and total.
- Choose adjustment mode.
- Review new A/B values.
- Apply changes to reach target.
Formula Breakdown
Adjust Both: A = Total × Ratio ÷ (1 + Ratio); B = Total − AThe "Adjust Both" mode is derived directly from the ratio definition: if A:B = Ratio, then A = Ratio × B, and since A + B = Total, substituting gives A = Total × Ratio ÷ (1 + Ratio) and B = Total − A.
Worked Example
Current A = 40, Current B = 60, Target Ratio = 1 (meaning a 1:1 split), Total Constraint = 120, Adjustment Mode = Adjust Both.
- New A: 120 × 1 ÷ (1 + 1) = 120 ÷ 2 = 60
- New B: 120 − 60 = 60
- Delta A: 60 − 40 = +20 (add 20 to A)
- Delta B: 60 − 60 = 0 (B is already at target)
If instead you chose "Adjust A Only" with the same current values and a target ratio of 1: New B stays at 60 (unchanged), and New A = 1 × 60 = 60, giving the same Delta A of +20 but leaving the total unconstrained (in this case it still happens to total 120, but with a different ratio the two modes would diverge).
Interpretation Guide
| Largest Required Change | Meaning | Action |
|---|---|---|
| 0–5 | Current split is already close to the target ratio. | Minor adjustment, low priority. |
| 6–15 | Moderate drift from the target ratio. | Plan the shift into your next normal adjustment cycle. |
| 16–30 | Large drift from the target ratio. | Consider staging the adjustment across multiple steps. |
| 30+ | Major drift from the target ratio. | Revisit whether the target ratio or total constraint is still realistic before committing to the full change. |
Common Mistakes to Avoid
- Using "Adjust Both" with an outdated Total Constraint. This mode ignores your current A+B sum and rebalances toward the Total Constraint you enter — if that total no longer reflects your actual combined A+B, the suggested deltas will move you toward the wrong total, not just the wrong ratio.
- Confusing Target Ratio direction. A Target Ratio of 2 means A should be twice B (a 2:1 split), not that A should be half of B — double-check which side of the ratio you intend before reading the results.
- Ignoring which Adjustment Mode you selected. "Adjust A Only" and "Adjust B Only" hold one side completely fixed, which can produce a very different total than "Adjust Both" — make sure the mode matches whether one side is actually fixed in your real situation (e.g., a fixed budget category) or whether both sides can move.
- Rounding before reading the delta. Use the Round To setting to control final precision rather than manually rounding your current A/B inputs first, since rounding inputs before calculating can shift the computed deltas slightly from the true unrounded values.
Measurement Notes
Treat this calculator as a directional planning instrument. Output quality improves when your inputs are anchored to recent real data instead of one-off assumptions.
Run multiple scenarios, document what changed, and keep the decision tied to trends, not a single result snapshot.
Related Calculators
- Ratio Step Down Calculator — step a ratio down gradually across multiple stages instead of one rebalance.
- Allocation Ladder Calculator — build a multi-tier allocation instead of a single two-way ratio.
- Weighted Range Midpoint Calculator — find a biased midpoint between two bounds instead of a two-value ratio.