Interest Rate Parity Calculator

The interest rate parity calculator estimates the exchange rate implied by a spot quote, a term in days, and the annual interest rates for the price and base currencies. It is built for the international-finance idea that exchange rates and interest rates should line up when investors can borrow, lend, convert, and hedge across currencies. In the covered version, the output is labeled as a no-arbitrage forward rate. In the uncovered version, the output is labeled as an expected future spot rate. The numerical formula is the same in the current calculation method.

This page is informational, not investment advice; FX is high-risk. Parity relationships are teaching and benchmarking tools. Actual trades face bid-ask spreads, collateral requirements, credit limits, settlement calendars, liquidity, taxes, capital controls, and execution risk. A parity rate can help you question a quote, but it cannot make a currency transaction risk-free.

Inputs and quote direction

Choose covered interest rate parity if you are comparing a forward contract with spot and money-market rates. Choose uncovered interest rate parity if you want the same rate relationship framed as an expected future spot rate. The spot exchange rate must be entered as price-currency units per one base-currency unit. For example, if one euro costs 1.1000 U.S. dollars, USD is the price currency and EUR is the base currency.

The forward term is entered in days. The price currency annual rate is the annualized rate for the currency in the numerator of the quote. The base currency annual rate is the annualized rate for the currency being priced. The form accepts rates as percentages. The calculation method rejects invalid numeric values, spot rates less than or equal to zero, days less than or equal to zero, and cases where the base-currency period denominator is less than or equal to zero.

Formula

The calculator converts the term to a 365-day year fraction:

$\text{year fraction} = \frac{\text{days}}{365}$

It then converts annual percentage rates to period rates:

$\text{price period rate} = \frac{\text{price annual rate}}{100} \times \text{year fraction}$

$\text{base period rate} = \frac{\text{base annual rate}}{100} \times \text{year fraction}$

The parity rate is:

$\text{parity rate} = \text{spot rate} \times \frac{1 + \text{price period rate}}{1 + \text{base period rate}}$

The displayed forward premium or discount is:

$\text{premium or discount} = \left(\frac{\text{parity rate}}{\text{spot rate}} - 1\right) \times 100$

The annual rate difference shown in the result panel is the price currency annual rate minus the base currency annual rate.

Worked example using the default inputs

The default form selects covered parity, uses a spot rate of 1.1000, a 90-day term, a price currency annual rate of 0.8 percent, and a base currency annual rate of 3.2 percent. The year fraction is 90 divided by 365, or 0.2465753425. The price period rate is 0.8 divided by 100, multiplied by that fraction, which equals 0.0019726027. The base period rate is 3.2 divided by 100, multiplied by the same fraction, which equals 0.0078904110.

The parity rate is 1.1000 multiplied by 1.0019726027 divided by 1.0078904110. That equals 1.0935413722. The calculator displays the no-arbitrage forward rate with up to four decimals, so the default result appears as about 1.0935. The premium or discount is 1.0935413722 divided by 1.1000, minus one, multiplied by 100. That equals about negative 0.587148 percent. The annual rate difference is 0.8 minus 3.2, or negative 2.4 percentage points.

How interest rate parity is used

Covered parity is used to compare quoted forwards with spot and money-market rates. If a quoted forward is far away from the parity benchmark, the difference may reflect transaction costs, balance-sheet constraints, credit charges, or a possible data issue. Large institutions monitor these relationships because they connect funding markets with FX swaps and forwards. Students use the same formula to understand why a currency with a higher interest rate often trades at a forward discount in a direct quote.

This calculator is close to, but not identical with, the currency forward calculator, which uses a 360-day convention and reports forward points. The cross exchange rate calculator can help validate the spot quote when it is derived indirectly from a third currency. The carry trade calculator shows the riskier trading idea that investors may pursue when interest-rate differentials are attractive.

Risks and interpretation tips

Parity can fail as a clean real-world arbitrage when traders cannot borrow at the assumed rates, face capital or collateral constraints, or pay different taxes and transaction costs. During stress, the availability of dollar funding, bank balance-sheet capacity, and demand for hedges can push forward and swap prices away from simple textbook parity. Uncovered parity is even less certain because it replaces a locked forward with an expectation about a future spot rate.

For careful use, keep the spot quote direction visible, confirm whether your interest rates are annual money-market, deposit, swap, or policy rates, and avoid mixing a 365-day parity result with a 360-day forward quote without adjusting. If you select uncovered parity, remember that the calculator changes only the label. The computed number is still the same mathematical parity rate.

Sources

Source version: issuer pages current when accessed July 9, 2026; no unstated effective year is assumed.

• BIS, Triennial Central Bank Survey of foreign exchange and OTC derivatives markets — FX swap, forward, and derivatives market background.
• Federal Reserve, Foreign Exchange Rates H.10 — spot exchange-rate reference context.
• European Central Bank, Euro foreign exchange reference rates — euro reference rates and FX data publication.