The planning number: what return you can actually count on, by horizon

Every goal plan has a hidden assumption buried in it: the return your money will earn between now and the deadline. Get that number wrong and everything downstream — the monthly contribution, the finish date, the confidence — is wrong with it. The most common error is grabbing the famous long-run stock average (about 10% nominal) and plugging it into a five-year goal, which is a bit like planning a road trip around your car's top speed. Averages describe the long run; goals live in specific, often short, windows. The fix is a planning number: a deliberately conservative return assumption that varies by horizon, plus a schedule for stepping risk down as the goal date approaches.

Why the planning number sits below the average

Three forces push your usable return below the headline average. First, volatility drag: a portfolio that averages 10% arithmetically compounds at more like 8-9% because losses hurt more than equal gains help. Second, sequence risk: over short windows, the order of returns matters enormously — a bad first two years on a five-year goal can be unrecoverable even if the average over the period ends up fine. Third, your goal has a hard date, and hard dates convert volatility from an annoyance into a threat. A retirement account can wait out a bear market; a tuition bill due in August cannot.

The gap between the two right-hand columns is not pessimism — it's a margin of safety. If markets deliver the average, you finish early or with a surplus, which is a pleasant problem. If they deliver a bad decade, the conservative assumption means your contribution rate was high enough to survive it. Plans built on optimistic numbers fail quietly for years and then loudly at the deadline.

The glide-down: de-risking on a schedule

A 12-year goal is allowed to be aggressive today, but it will be a 3-year goal eventually, and it needs to be invested like one by then. The mechanism is a glide path: a pre-committed schedule for shifting from stocks toward cash as the date approaches. Pre-committed is the operative word — deciding in advance removes the temptation to 'wait for the market to recover' in the exact years when waiting is most dangerous.

1. 1
   10+ years out: growth mode

   70-90% stocks. Volatility is your friend here — contributions buy more shares in down years, and there's time to recover from anything short of a lost decade.

2. 2
   5-7 years out: first step down

   Shift to roughly 50-60% stocks. New contributions start going to the bond/cash side. Update the plan using the shorter horizon's planning number — the required contribution usually rises a little.

3. 3
   3 years out: capital preservation

   Cut stocks to 20-30% or less. A 20% drawdown from here can no longer be earned back by contributions before the deadline. Each strong market month is an opportunity to sell down, not a reason to ride longer.

4. 4
   12-18 months out: done investing

   Move to cash, T-bills, or CDs maturing before the need date. The goal's return engine is finished; the only job left is showing up in full.

Putting the number to work

1. Write down each goal's dollar target and need date, then read its planning number off the horizon table above.
2. Solve for the monthly contribution using that number — any goal calculator works; the assumption matters far more than the tool.
3. Set the starting allocation to match the horizon, and put calendar reminders on the glide-down trigger dates (7 years out, 5, 3, 18 months).
4. Re-run the math once a year: horizons shrink, so both the planning number and the allocation should drift more conservative even between glide steps.
5. Treat any surplus from good markets as arriving early, not as license to raise the target mid-flight.

The bottom line

The return you assume is a decision, not a fact, and it should shrink as the horizon does: roughly 6-7% for 10-plus-year goals, 5% for the middle distance, 4% and falling once the date is inside five years, cash-only returns in the final stretch. Pair the conservative number with a pre-committed glide-down and your goals stop depending on the market cooperating in any particular year. Plan low, glide down early, and let good markets make you early instead of letting bad ones make you late.