GDP growth calculator

Previous GDP of zero is rejected with invalid_input because the rate is undefined.

What GDP growth means and how the decimal rate is defined

The GDP growth rate is the relative change in gross domestic product between two periods. A later GDP above the earlier GDP is expansion; a later GDP below the earlier GDP is contraction; identical levels mean the measured economy was unchanged on this measure. The previous level is the denominator, so it cannot be zero: dividing by zero is undefined, and this calculator rejects that case instead of returning Infinity or NaN. If previous_gdp is the level in the earlier period and current_gdp is the level in the later period, the identity is growth_rate equals current_gdp minus previous_gdp, divided by previous_gdp. That result is a decimal, not a percent: a rise from one hundred to one hundred three is 0.03, which a journalist would quote as three percent only after multiplying by one hundred. Current GDP may sit above, equal to, or below previous GDP, including zero, because a collapse of measured output is a real contraction of one hundred percent, which is a growth_rate of negative one. This tool does not annualize a multi-year gap, does not convert nominal GDP into real GDP, does not rebase one series onto another, and does not fetch official numbers from a statistical office. It evaluates the closed form on the two numbers you already have from a release, a textbook problem, or a spreadsheet. It is not an expenditure-approach GDP identity that sums consumption, investment, government spending, and net exports, and it is not a compound annual growth rate over n years: those are different searches and different jobs.

Input contract, response shape, and error behaviour

The value must be a finite number and must not be zero. Failed validation does not consume balance. POST a JSON body with two required finite numbers in the same currency and the same units. previous_gdp is the gross domestic product of the earlier period and is the denominator of the growth rate; aliases previous, gdp_previous, gdp0, earlier_gdp, and gdp_t0 are accepted so spreadsheets can keep familiar column names. If previous_gdp is missing, empty, infinite, or exactly zero, the task fails with invalid_input naming previous_gdp; a zero previous period is the contracted error and it is not billed. current_gdp is the gross domestic product of the later period; aliases current, gdp_current, gdp1, later_gdp, and gdp_t1 map to the same field. It must be a finite number and may be higher, equal, or lower than the earlier print. Each field may arrive as a JSON number or a numeric string. On success the body echoes previous_gdp and current_gdp, returns change as current minus previous rounded half-up to ten decimal places, returns growth_rate as the decimal rate rounded half-up to ten decimal places, sets direction to expansion, contraction, or unchanged according to the sign of the rate, and includes a formula string restating the identity. Example: previous one hundred and current one hundred three yields growth_rate 0.03, change three, and direction expansion. Previous twenty thousand and current eighteen thousand yields growth_rate negative 0.1 and direction contraction.

Where a GDP growth calculator earns its keep

Newsrooms, research desks, and classrooms use a GDP growth calculator when a statistical release prints two GDP levels and the question is the relative change, not a reconstruction of the expenditure identity that produced those levels. Households and students compare last year’s GDP with this year’s to see whether the measured economy expanded or contracted, without converting the decimal into a percent until they write the sentence. When you later need the level of GDP rather than its growth, or a multi-year compound rate, use those dedicated identities rather than stretching this two-print GDP growth contract. Product teams wire the same contract into briefing tools so a reporter who types two official prints sees the identical growth_rate the API will later return in a test suite. Keep requests small, pin the rounded growth_rate in tests, and you get reproducible decimal growth for worksheets, LMS plugins, and edge workers without standing up a national-accounts client of your own.