Contribution vs. Return Sensitivity
Know which dial moves your plan more
When a projection comes in lower than you hoped, there are two ways to fix it: save more or assume a better return. One of those is a decision, the other is a wish. This guide shows you how to tell which one is actually carrying your plan.
The three-run experiment
Start with a baseline: $5,000 starting balance, $300 a month, 7% return, 25 years. Note the ending number.
Run two: change the return to 8% and nothing else. Note the new number and the difference.
Run three: put the return back to 7% and raise the contribution to $400. Compare that difference with the one from run two.
- A 1% higher return costs you nothing and is entirely outside your control.
- An extra $100 a month costs real money and is entirely inside your control.
- At mid-length horizons, $100 a month often rivals a full point of return.
- At very long horizons, the return assumption slowly pulls ahead of the contribution.
- Only one of the two shows up in your bank account whether markets cooperate or not.
Why the horizon flips the answer
Contributions add money linearly: every month you deposit the same amount. The return multiplies whatever is already there, and multiplication needs time and a big base to overtake addition.
So on a 10-year run, contributions dominate almost completely. On a 40-year run, the rate assumption starts to matter more than the monthly amount.
Test it. Run the same $100 versus 1% comparison at 10 years, then at 40 years, and watch the ranking change.
The third dial people forget
Contribution and return get all the attention, but time is usually the strongest lever on the page and the cheapest one to test. Take your baseline and add five years without touching anything else. On a 25-year run at 7%, those five extra years typically beat both the $100 contribution increase and the full point of return.
The catch is that time is only available if you have it. A 30-year-old can add five years by planning to retire at 67 instead of 62. A 58-year-old cannot manufacture a decade out of nothing, which is why the same comparison gives different advice at different ages.
Run the three-dial test at your own age and horizon rather than accepting a general rule. The ranking of contribution, return, and time genuinely changes depending on where you are standing.
How to use this when you set a goal
If you need a specific ending number, solve for the contribution first at a modest return like 6%. If the required monthly amount fits your budget, you have a plan that does not depend on a great decade.
Only after that should you rerun at 8% or 9%, and read the difference as upside rather than as the plan itself.
This ordering matters because a plan built on 10% returns fails quietly. You will not notice it failing until you are ten years in and behind.
Try these inputs
- Baseline: $5,000 start, $300/mo, 7%, 25 years
- Return bump: same inputs at 8%
- Savings bump: $5,000 start, $400/mo, 7%, 25 years