Marginal propensity budgeting: a formula for every new dollar

Economists have a term for what fraction of an additional dollar of income a person spends: the marginal propensity to consume. Empirically, most people's is close to 1.0 — within a year of a raise, nearly the whole thing has been absorbed into lifestyle, often without a single deliberate decision. Marginal propensity budgeting turns this descriptive concept into a prescriptive tool: instead of letting your marginal propensity to consume happen to you, you choose it, write it down as a formula, and apply it mechanically to every dollar of income above your current baseline. Your existing budget governs your existing income; the formula governs everything new.

The core insight is that new income is the cheapest money you will ever save. Cutting $400/month from a lifestyle you already live requires felt sacrifice every single day. Allocating $400/month from a raise you haven't received yet requires nothing — you've never tasted that money, so directing 60% of it to savings costs zero units of experienced deprivation. A household that merely holds lifestyle flat during three raises achieves what a decade of frugality lectures cannot.

Setting your marginal rates

The formula is a set of percentages that sum to 100, applied to the after-tax value of any income increase. A defensible default for a household with its emergency fund built and no high-interest debt: 50% to investing, 20% to named medium-term goals, 10% to giving or family, and 20% to permanent lifestyle growth. The lifestyle share is not a leak — it's load-bearing. A formula with 0% lifestyle growth gets abandoned at the second raise; 20% is the price of a rule you'll still follow at 45. Households still carrying expensive debt or a thin emergency fund should run a temporary override — 70%+ of marginal dollars to the deficit until it's cleared — then revert to the standing formula.

Different dollars, different formulas

Not all new money is the same shape, and the formula should acknowledge it. A raise is permanent and recurring — its allocation should also be permanent and recurring, implemented as changed automatic transfer amounts within 30 days of the first new paycheck. A bonus is a one-time lump — same percentages, applied once, executed the week it lands. A true windfall (inheritance, home sale proceeds, equity liquidity) is large enough that the standing formula needs a preamble: park it in a HYSA for 90 days, settle the emotional weather, then apply a windfall-specific split that typically weights investing and goals more heavily, because a $150,000 event shouldn't buy 20% — $30,000 — of permanent lifestyle inflation off a single occurrence.

1. Raise: recompute after-tax monthly delta, apply the formula, change your automatic transfers before the second new paycheck arrives.
2. Bonus: apply the formula to the net amount in one sitting; the lifestyle share is a purchase, not a raised baseline.
3. Windfall over ~6 months of expenses: 90-day cooling period in cash, then a written one-page plan using shifted weights (e.g., 65/25/5/5 invest/goals/giving/lifestyle).
4. Side income: decide whether it's income (run the formula) or seed corn (reinvest in the side venture) — but decide once, not per payment.

Why a formula beats deciding each time

Every windfall arrives with a persuasive story about why it's special — this bonus was for a brutal year and deserves to be enjoyed; this raise merely catches up to what you were owed; this inheritance is emotionally complicated. Case-by-case deciding means negotiating with the most motivated salesperson you know: yourself, at the exact moment of maximum temptation. A pre-committed formula moves the decision to a calm moment and reduces every future windfall to arithmetic. This is the same logic as automatic 401(k) escalation, generalized to your entire financial surface. The formula also ends the characteristic couple's fight over found money, because the answer to 'what should we do with it?' was signed by both parties years earlier.

• Decision cost drops to zero: the formula runs in minutes, and the mental bandwidth stays available for actual life.
• Lifestyle inflation becomes chosen, bounded, and guilt-free — 20% of every raise, forever, is a lot of nice things.
• Savings rate rises automatically with income, which is precisely when it should rise: your marginal dollars are your most saveable dollars.
• The formula is auditable: once a year you can check whether the transfers actually match the percentages.

The annual true-up

Once a year, reconcile. Take total gross income growth since your baseline year, compute its after-tax value, and compare it to the growth in your automatic saving and investing. If income is up $18,000 net over three years and automated savings are up only $4,000/year, your realized marginal propensity to consume was about 78% — far from the 50% you intended. No blame, just recalibration: raise the transfers to where the formula says they should sit, or consciously amend the formula if your priorities genuinely changed. The point of writing the rule down was never perfection; it was making the drift visible enough to correct.

The bottom line

Your budget governs the income you have; marginal propensity budgeting governs the income you're about to get — which, over a career, is most of it. Pick percentages that match your stage, give lifestyle its honest 20% so the rule survives contact with being human, apply heavier weights to true windfalls, and enforce everything through automatic transfers changed within 30 days. The households that end up wealthy on ordinary salaries aren't the ones that cut the deepest; they're the ones that decided, in advance and in writing, what every new dollar would do.