How this instrument works
Marginal propensity to save is a slope, not a snapshot: it measures how much of the next dollar of income gets saved, not how much of total income already sits in a savings account. Divide the change in savings by the change in income that produced it — ΔS ⁄ ΔY — and the result is a rate of change, exactly the way John Maynard Keynes framed it in his 1936 theory of consumption and saving, where every extra unit of income splits between spending and saving in a fixed proportion at the margin.
The number matters most through its reciprocal. Divide 1 by the marginal propensity to save and you get the textbook Keynesian spending multiplier — the theoretical amount a single new dollar of spending expands through an economy as it gets earned, partly saved, partly respent, and earned again downstream. A government economist sizing up a stimulus package, a forecaster modeling how a tax cut ripples through consumer spending, and a macro student checking a textbook problem all reach for this same ratio before reaching for the multiplier it implies.
The ratio only holds for the specific income change it was measured over — a household's marginal propensity to save near the poverty line behaves nothing like the same household's marginal propensity to save on an unexpected bonus once savings and credit access widen. It also says nothing about where the saved dollar goes: a retirement account, a checking-account cushion, and a stock purchase all count identically as savings here, and the formula has no view on which choice serves the saver best.
- Enter the extra amount set aside into "Change in savings, $" — the dollar shift in savings between the two income levels being compared.
- Enter the income change that produced it into "Change in income, $" — the denominator the ratio is measured against.
- Read "Marginal propensity to save" — the share of that additional income that was saved rather than spent.
- Subtract the result from 1 to recover the matching marginal propensity to consume for the same income change.
- Invert the result — divide 1 by it — to see the Keynesian spending multiplier that saving rate implies.
Worked example — $200 saved from a $1,000 raise
Take an extra $1,000 in income — a raise, a bonus, a one-time payment — and suppose $200 of it lands in savings while the rest gets spent. Divide 200 by 1,000 and the marginal propensity to save comes out to exactly 0.2: one-fifth of every additional dollar earned was saved rather than put toward goods and services.
That 0.2 is the mirror image of an 0.8 marginal propensity to consume on the same $1,000, since the two must sum to 1 — spend it or save it, there is no third bucket. Invert the 0.2 and the implied Keynesian multiplier is 5: in the simplified textbook model, new spending that behaves like this household's marginal habits could expand economic activity fivefold before the leakage into saving fully dissipates it.
Questions
What does a marginal propensity to save of 0.2 mean in practice?
It means 20 cents of every extra dollar earned gets saved and the remaining 80 cents gets spent, for the specific income change being measured. It says nothing about a household's total savings balance or its overall savings rate — only what happens to the next dollar that arrives.
How is this different from an average savings rate?
An average savings rate divides total savings by total income across an entire period — a photograph of one point in time. Marginal propensity to save divides only the change in savings by the change in income that caused it — a slope, describing what happens to new money rather than to income already earned and allocated.
Why do economists convert this ratio into a multiplier?
Because 1 divided by the marginal propensity to save gives the theoretical ceiling on how far a single new dollar of spending can expand through an economy as it gets earned, partly saved, and partly respent at each round. Forecasters use that multiplier to estimate how much a stimulus payment or tax cut moves total output, not just one household's budget.
Can marginal propensity to save be negative or above 1?
Yes, in either direction. A negative value means savings fell even as income rose — dissaving, often paying down debt or drawing on credit. A value above 1 means savings grew by more than income did, which happens when a windfall gets banked in full while older spending gets cut at the same time.
Does marginal propensity to save always equal 1 minus marginal propensity to consume?
Yes, for the identical income change being measured, because every extra dollar of income has only two places to go — spent or saved — so the two propensities are defined to sum to exactly 1. If your two figures do not sum to 1, they were computed over different income changes.
Is marginal propensity to save the same figure for every household?
No. It typically runs low for households near subsistence, where nearly every added dollar gets spent on necessities, and it often runs higher for households with more room to set money aside. The ratio describes one income change for one household or group — it is not a fixed constant across an economy.
References
- U.S. Bureau of Economic Analysis — Personal Saving Rate data
- Board of Governors of the Federal Reserve — Financial Accounts of the United States
Read this first: This instrument shows arithmetic, not advice. Real offers add fees, taxes and terms that vary by lender and place — verify the figures against your actual paperwork before deciding anything.