How the MPC Calculator works
The marginal propensity to consume (MPC) is a core concept from Keynesian economics that measures how much of every extra dollar of income a household, group, or economy spends rather than saves. This calculator applies the standard textbook definition to whatever income and consumption figures you enter, then derives the marginal propensity to save and the simple spending multiplier that follow directly from it.
The formula
MPC is the change in consumption divided by the change in income:
MPC = ΔC / ΔY
where ΔC is new consumption minus initial consumption, and ΔY is new income minus initial income. Enter an initial and a new value for income and for consumption, and the calculator computes both deltas and divides them for you.
Worked example
Suppose income rises from $50,000 to $60,000 (ΔY = $10,000) and consumption rises from $45,000 to $52,000 (ΔC = $7,000). Then MPC = 7,000 / 10,000 = 0.70. In other words, about 70 cents of every additional dollar of income was spent, and the remaining 30 cents was saved.
MPS and the spending multiplier
The marginal propensity to save, MPS, is simply the leftover share of new income: MPS = 1 − MPC. Because every extra dollar is, in this simple framework, either spent or saved, MPC and MPS always add to 1.
In the simple Keynesian multiplier model, an initial injection of spending circulates through the economy as each round of recipients spends a share (the MPC) of what they receive. Summing that geometric series gives the spending multiplier:
Multiplier = 1 / (1 − MPC) = 1 / MPS
With MPC = 0.70, the multiplier is 1 / 0.30 ≈ 3.33 — a common textbook illustration of why a higher MPC amplifies the effect of stimulus spending more than a lower one.
Reading the range
MPC normally falls between 0 and 1. Lower-income households, which have less room to save, tend to show an MPC closer to 1 (they spend nearly all of an extra dollar). Higher-income households, or economies during uncertain times, often show a lower MPC as more of each new dollar goes to savings. A calculated MPC below 0 or above 1 is unusual — it typically means consumption moved in the opposite direction from income, or grew by more than the income change itself, and is worth double-checking against the source data before drawing conclusions.
Assumptions and limits
This calculator performs the standard two-point MPC calculation from a single before/after pair of income and consumption figures; it does not estimate MPC from a full time series or regression, and it does not account for taxes, transfers, or the distinction between average and marginal propensities across an entire population. Real-world MPC estimates used in economic research typically come from panel data or natural experiments and can vary by income level, wealth, and how temporary or permanent the income change is expected to be. Treat this tool as a way to compute the textbook ratio from your own numbers, not as a substitute for empirical estimation.