Method and sources
How DepositFX calculates a two-currency comparison
This page explains the calculation model, the direction of the exchange rate, the role of the reference table and the limits of the result. It is an editorial explanation of the tool, not a bank quote or a recommendation.
1. What the calculator is comparing
DepositFX compares two hypothetical ordinary fixed-deposit paths from the same starting amount. The base-currency path keeps the money in the currency already held. The comparison path converts the amount, applies a deposit rate and term, and converts the maturity value back. The terms may differ, so the result panel explicitly warns when the comparison is not like-for-like. This tool does not model structured dual-currency investments, strike levels, conversion obligations or embedded options.
2. Calculation model
The current calculator uses simple interest. For each path, the estimated interest is the principal multiplied by the annual rate and the term in months divided by 12.
base maturity B = P × (1 + rᵦ × mᵦ ÷ 12)comparison principal Q = P ÷ R₀comparison maturity T = Q × (1 + rₜ × mₜ ÷ 12)estimated difference Δ = T × (R₀ − S) − Bdisplayed break-even quote R* = B ÷ T + SP is the starting amount in the base currency; r is an annual rate written as a decimal; m is the term in months; R₀ is the base-currency amount for one unit of comparison currency; and S is the optional maturity conversion adjustment in the same quote units. The calculator uses simple interest and applies S once to the maturity conversion quote. It does not model separate entry and exit quotes or compound interest.
3. Reproducible example with the threshold shown
Assume P = CNY 100,000, both terms are 12 months, the CNY rate is 1.50%, the USD rate is 4.00%, R₀ = 7.20 CNY per USD, and S = 0. The base maturity is CNY 101,500. The USD principal is USD 13,888.89 and its maturity value is USD 14,444.44. At 7.20, the comparison path converts back to CNY 104,000, an estimated difference of +CNY 2,500.
The break-even quote is 101,500 ÷ 14,444.44 = approximately 7.0269 CNY per USD. At 7.00, the converted result is about CNY 101,111.11, or CNY 388.89 below the base path. At 7.40, it is about CNY 106,888.89, or CNY 5,388.89 above. These are arithmetic scenarios, not exchange-rate forecasts. The break-even guide shows how to interpret the same line when rates, terms or quote direction change.
4. Exchange-rate direction and spread
The calculation stores the quote as base currency per one comparison-currency unit. A displayed quote such as “1 USD = 7.20 CNY” is therefore 7.20 when CNY is the base currency and USD is the comparison currency. The direction switch displays its reciprocal but preserves the same underlying calculation. In that inverse display, a higher threshold in the shown units corresponds to a lower base-per-comparison quote.
The optional adjustment S is subtracted once from the maturity conversion quote. It is not the sum of a bank's entry and exit spread and does not model two separate trades. For an exact round trip, use the actual initial conversion quote to calculate the comparison-currency principal and the actual exit quote to convert maturity proceeds, then subtract transaction charges separately. See the exchange-cost guide.
5. Reference rates and source policy
The home-page table contains one selected public 12-month product for each currency; it is neither a market average nor a “best rates” ranking. Product types differ: examples include an Australian standard term deposit, a Canadian non-redeemable GIC, a UK fixed-rate bond, and a Singapore indicative board rate. The source note identifies selected balance tiers and excludes an expired promotion where an ordinary rate is available.
“Checked” means the official page was reviewed on 8 October 2026; it is not necessarily the rate's effective date. Some pages publish tiered balances or customer-specific offers, while bank rates may change without notice. The values are comparison inputs, not offers or promises of eligibility. Confirm the current rate, balance, channel, customer status, interest timing, fees and early-withdrawal rules on the linked page. Report a broken link or a clearly changed figure through the contact page.
6. What is outside the model
- Taxes, withholding, account fees, transfer fees and intermediary charges.
- Deposit insurance limits, customer eligibility and product availability.
- Early withdrawal, renewal, compounding and changing rates during the term.
- Slippage between the displayed exchange rate and the rate actually executed.
- Any prediction of where a currency or interest rate will move next.
7. Editorial updates
The calculator logic and explanatory pages are maintained as a small independent project. We aim to keep formulas visible, distinguish estimates from facts, link to primary bank pages and show checked dates for reference rates. Changes to calculation logic or source coverage should be described in the page or repository rather than hidden behind an unexplained result.
8. Input and comparison rules
Amount must be a finite value greater than zero. Deposit rates may be zero or higher. Terms must be positive whole months. The exchange rate must be greater than zero, and the optional maturity adjustment must be zero or greater but smaller than the current quote. Invalid text, negative values and blank required fields stop the calculation and clear prior results.
A reference rate is used only when the annual-rate field is blank and a currency-and-term match exists. A typed but invalid rate is never replaced silently. A reference value for 12 months is not reused for another term. If the two terms differ, the page labels the output as a comparison across different time horizons.
For a reproducible check, record amount, both rates and terms, base and comparison currencies, quote direction, exchange rate, maturity adjustment and the reference source date. Re-entering those values should reproduce the same simple-interest estimate.