Marginal Propensity to Consume Calculator
From the change in consumption and the change in disposable income, compute the marginal propensity to consume: how much of each extra dollar is spent.
Input Data
Results
At a glance:The marginal propensity to consume (MPC) is the fraction of an extra dollar of disposable income that is spent. MPC = ΔC ÷ ΔY, where ΔC is the change in consumption and ΔY the change in disposable income. MPC plus MPS (marginal propensity to save) equals 1.
Formula
MPC = change in consumption ÷ change in disposable income.
$$MPC = \dfrac{\Delta C}{\Delta Y}$$$$Multiplier = \dfrac{1}{1 - MPC}$$How to Use
- Enter the change in consumption (ΔC).
- Enter the change in disposable income (ΔY).
- Read the MPC.
FAQ
What is MPC and why is it important in economics?
MPC (marginal propensity to consume) measures 'how much of each extra dollar of disposable income people spend'. It is a core Keynesian concept for understanding consumer behaviour and aggregate demand. Defined as MPC = change in consumption ÷ change in disposable income (ΔC ÷ ΔY), it lies between 0 and 1. Example: if your monthly disposable income rises by HK$800 after a raise and you spend HK$600 of it, your MPC is 0.75. It matters because it links individual spending decisions to the whole economy: in Keynesian theory, total demand depends heavily on consumption, which depends on income and MPC. When the government wants to stimulate the economy (vouchers, tax cuts), how much extra spending results hinges on the beneficiaries' MPC — the higher it is, the more of the money is spent and recirculated, the stronger the boost. MPC is also the basis of the spending multiplier (multiplier = 1 ÷ (1 − MPC)), which determines how much autonomous spending is amplified.
What is the relationship between MPC and MPS?
MPC and MPS (marginal propensity to save) are two sides of the same coin and always sum to 1 — MPC + MPS = 1. The reason is simple: when disposable income rises, the extra money can only go to consumption or saving — whatever is not spent is, by definition, saved. So the share spent (MPC) plus the share saved (MPS) must equal 100%. Example: if MPC = 0.75, then MPS = 0.25, and the two add to exactly 1. This is very handy: knowing one gives you the other (MPS = 1 − MPC). In economic analysis they describe the same income allocation from two angles. It also affects the multiplier: spending multiplier = 1 ÷ (1 − MPC) = 1 ÷ MPS, so a higher MPC (lower MPS) means a larger multiplier.
Why does MPC differ across people and income levels?
MPC is not fixed; it varies with income level, financial situation and the economic environment — and that variation matters for policy design. Generally, lower-income households have a higher MPC than higher-income ones. Low-income families spend most income on necessities (food, rent, transport), leaving little room to save, so extra income mostly goes to needed consumption, pushing MPC close to 1. High-income households have their needs met, so extra income is more likely saved or invested, giving a lower MPC. The policy implication: to stimulate consumption via 'more disposable income' (subsidies, tax cuts), targeting higher-MPC groups (usually lower- and middle-income) is more effective — they spend a larger share back into the market, producing a stronger multiplier; money flowing to high-MPC, low-income recipients yields more immediate stimulus than to high-income, low-MPC ones. Beyond income, MPC is also shaped by optimism/pessimism about the future, precautionary saving needs and the wealth effect.
How does MPC differ from APC (average propensity to consume)?
Both describe 'the share of income spent on consumption', but one looks at the marginal (increment) and the other at the average (stock), so they differ in meaning and usually in value. APC = total consumption ÷ total disposable income — the overall share of all income spent. MPC = change in consumption ÷ change in income (ΔC ÷ ΔY) — how much of each extra dollar is spent. Example: someone earning HK$20,000 and spending HK$16,000 has APC = 0.8. After a HK$5,000 raise, if they spend an extra HK$3,000 and save HK$2,000, the raise's MPC = 3,000 ÷ 5,000 = 0.6. So the same person can have APC 0.8 but MPC 0.6. Why the gap? Under the Keynesian consumption function, consumption has two parts: autonomous consumption (basics you must pay even with no income) and induced consumption (rising with income). Because of the autonomous 'base', at low income APC can be high or even above 1, and as income rises APC falls toward MPC. Generally: (1) APC is usually above MPC; (2) APC falls and approaches MPC as income rises; (3) MPC is relatively stable, APC varies with income. Use APC for the overall consumption/saving structure; use MPC for the marginal effect of extra income or fiscal stimulus.
Why is the spending multiplier 1 ÷ (1 − MPC), and how does the chain work?
The spending multiplier is an elegant Keynesian result: one injection of autonomous spending ultimately drives total output to several times the original amount, and that multiple equals 1 ÷ (1 − MPC). The key is the 'consumption chain'. Imagine the government adds HK$100 of autonomous spending (e.g. a construction project paying a contractor HK$100). That HK$100 becomes the contractor's income. With MPC = 0.75, they spend HK$75 (100 × 0.75) on consumption, which becomes the income of the merchants they buy from. Those merchants spend 75% of HK$75 (HK$56.25), which becomes the next group's income, and so on. Each round is MPC times the previous, forming a declining geometric series: 100 + 75 + 56.25 + 42.19 + … Its sum = 100 ÷ (1 − 0.75) = 100 ÷ 0.25 = 400. So HK$100 of spending, through the consumption chain, drives HK$400 of output — a multiplier of 4, exactly 1 ÷ (1 − MPC). The higher the MPC, the longer the chain lasts and the larger the multiplier; the lower the MPC (more saving), the faster the leakage and the smaller the multiplier. Since 1 − MPC = MPS, the multiplier can also be written 1 ÷ MPS. In reality, taxes and imports are also leakages, so the actual multiplier is smaller than this pure value. This tool is for educational estimation only.
Related Tools
References
Content review: Calculatorism Finance Team. Results are for reference only; please refer to the relevant authorities for the official figures.