Present value tab (labelled PV): the value today of amounts due on future dates, at one discount rate.

• Inputs: a valuation date, a discount rate (% p.a., may be negative), compounding (yearly by default, half-yearly, quarterly, monthly, daily, continuous or none) and a day count basis (Actual/365 by default, Actual/360 or Actual/Actual). Amounts are in HK$.
• Cash flows: any number of date + amount rows with an optional description. A minus sign (or brackets) marks money paid out, so the total is a net present value. An amount dated before the valuation date is grown forward to it at the same rate, and the page says so.
• Repeating cash flows: one row for an amount every month, quarter, half-year or year, from a first date, for up to 1,200 times (e.g. rent or instalments). It becomes a dated row for each time, e.g. "Rent (3 of 12)", keeping the first date's day of the month where the month has it (31 Jan, 28/29 Feb, 31 Mar, ...). The form shows the last date as you type.
• Solve for the rate (IRR): instead of entering a rate, find the rate at which the cash flows are worth zero on the valuation date, under the compounding and day count chosen. It needs money paid out and money received, and looks from −99% to 1,000% p.a. With Actual/365 and yearly compounding it's Excel's XIRR (Microsoft's example gives 37.336253%, as Excel does). If more than one rate works (cash flows that switch between paid and received more than once), it shows the one closest to 0% and lists the others. Switching back to Present value starts from the rate found.
• Timing: dates or periods. Periods (T0, T+1, T+2, ...) put each cash flow at a whole period from now instead of a date, with each period a year, half-year, quarter or month. The rate is still % p.a. and the page shows it a period (e.g. 8% p.a. = 2% a quarter), so nobody divides by 4 or 12 by hand; the discount factor at T+n is (1 + rate ÷ periods a year)^(−n). With yearly periods this is Excel's NPV (whose first value is T+1) and IRR (whose first value is T0): Microsoft's examples give 1,188.44, −2.1% and 8.7%, as in Excel. An IRR by periods is quoted a year (the rate a period × periods a year) and a period, plus the yearly rate with compounding. Links carry tm=p&pl=quarter, with the period number in place of the date.
• Discount factor: (1 + rate ÷ m)^(−m × years) for m compounding periods a year; daily (1 + rate ÷ year days)^(−days); continuous e^(−rate × years); none 1 ÷ (1 + rate × years). Years = days ÷ 365 (or 360); under Actual/Actual, the days in each calendar year ÷ 365 or 366, added up.
• Results: the present value, the total of the amounts and the discount, plus a row per cash flow with its days, years, discount factor, present value and the working, e.g. 100,000.00 ÷ (1 + 5.000%)^(731 ÷ 365). Rows keep full precision; only the totals are rounded to cents.
• Actual/365 with yearly compounding matches Excel's XNPV with the valuation date as its first date.
• Downloads: PDF and Excel, each with the inputs, totals, every row and its working, and the disclaimer. In Excel the valuation date and the rate are cells at the top, and each row's days, years, discount factor and present value are live formulas that use them (Actual/Actual writes its 1 January splits as DATE(...)), with the totals as SUMs.
• Also: a shareable link (?tab=pv&v=...&r=...&cf=date,amount,description; a repeating one is rf=date,amount,every,times,description, and s=irr solves for the rate), and Recent and Saved like the other tabs.

Time Value of Money calculator (PV tab, switch at the top: Cash flows | Time Value of Money; N, I/Y, PV, PMT, FV): the financial-calculator equation, as on a BA II Plus or HP 12C. Choose what to solve for and fill in the other four.

• No ×12 or ÷12: choose how often payments are made (monthly, quarterly, half-yearly, yearly); enter the term in years or as a number of payments and the rate as % p.a. The form shows the conversions as you type (e.g. "= 360 monthly payments", "= 0.416667% a month").
• Signs: + for money received, − for money paid (a loan you receive is PV +, its payments PMT −). Wrong signs get a message saying so.
• Equation: PV (1+i)^N + PMT (1 + i·due) ((1+i)^N − 1) ÷ i + FV = 0, where i = (1 + rate ÷ C/Y)^(C/Y ÷ P/Y) − 1, so compounding can differ from payments (e.g. half-yearly compounding, monthly payments); payments can be at the start of each period. N comes from logs; I/Y is found numerically and closest to 0%.
• Examples: one click fills and works out a loan payment, a savings goal, how long to repay, what rate, and a future sum's value today.
• Results: the answer (also filled into its box), total of payments, interest earned or paid, the rate with compounding, and a year-by-year table of payments, interest and balance. A term that isn't a whole number of payments ends with a smaller last payment. Shareable link (?tab=pv&m=tvm&ts=pmt&py=12&n=30&r=5&pv=1000000&fv=0), Recent and Saved.

Inflation tab: what an amount at one time is worth at another by the Hong Kong Composite Consumer Price Index from the Census and Statistics Department, table 510-60001, through its open data API.

• Calculator, in the same style as the other tabs (Calculate and Reset): Amount (HK$), In (when the amount is from) and Worth in (when to value it), either way round: HK$100 in 2000 is worth HK$156.28 now; HK$100 now was worth HK$63.99 in 2000. Each is Now | Year (Now = the latest month published), the year typed or stepped with − and +; Swap turns the question round. By: Year | Month: by year (the default) each year is C&SD's yearly average; by month a month box appears next to each year (months without figures greyed out). Shortcuts under In set 1, 5, 10, 20 or 30 years ago (that whole year, or by month the same month that many years before the latest) worth now; two times that overlap ask for two different times. Value = amount × CPI(worth in) ÷ CPI(in), shown in the first result box with the change in prices, the average a year, (CPI ratio)^(1 ÷ years) − 1 (a year counting from its middle), and the CPI at each end.
• Price level chart: the monthly Composite CPI rebased so a start month you pick = 100, up to the latest month.
• History, back-filled in full: monthly from October 1980 (index, year-on-year and month-to-month) and yearly from 1981, as tables and a chart of year-on-year inflation each month (5, 10, 20 years or all). The rates of change are C&SD's published figures; before October 2020 C&SD worked them out on the index base in use at the time, so they can differ slightly from changes in today's rebased index (published to 1 decimal place).
• Header: the latest year-on-year inflation, e.g. "HK inflation 1.7% Aug 2026". The rates ("Rates updated as at ..." and today's rates) sit at the top of each tab's panel, joined to its card, and show only what that tab uses: Interest (judgment debt, HSBC prime, US prime), Mortgage (HSBC prime, 1M and 3M HIBOR), Inflation (HK inflation) and none on PV. The tab bar stays put under the title and its one-line description.
• Daily update: scripts/update-cpi.mjs (run by npm run fetch-rates, or on its own) downloads the whole series, checks it (500+ months, no gaps, plausible figures, each full year's average of the months matching C&SD's yearly index, latest month no more than 4 months old) and rewrites site/cpi.json when it changes. The page only uses it on this tab, so an outage is a warning and the saved figures stay.
• Also: a shareable link (?tab=inflation&a=100&in=2000&w=now; months as 2000-01; now keeps following the latest month; older f / t / d=back links still open), Recent and Saved.

Advanced settings (collapsed by default, so the basic form stays simple):

• Day count basis (Actual/Actual by default) and rounding (round the total only, by default).

• Days counted (next to the dates, not under Advanced settings): End date included (the default, and usually how it is counted in court: both the start and end dates earn interest), or End date not included (as in Excel and the original workbook: the start date earns interest, the end date doesn't). With the end date included each period shows its last day counted (1 January to 31 March is 90 days), the same start and end date is one day, and the downloads follow (in Excel, days = end − start + 1).

• Calculation rows: Detailed (each published rate), the default, with a row for each published rate (even when a quarter repeats the same rate), payment and new year; or Combined (per rate period), which combines consecutive rows at the same rate, including rows split only by a republished rate, a new year (Actual/Actual), a payment, principal added or compounding, and shows the working as one sum, e.g. 100,000.00 × 8.000% × (184 ÷ 365 + 182 ÷ 366). Totals don't change. The downloads follow the same choice; in Excel a combined row is one live formula summing its pieces.

• Compounding: None (simple interest, the default), monthly, quarterly, yearly, daily or continuous.

  • Monthly / quarterly / yearly: unpaid interest is added to principal on each compounding date. Compounding dates can be:
    • From the start date (default): anniversaries of the start date. For a start on 31 Jan, monthly dates are 28/29 Feb, 31 Mar, and so on.
    • Calendar period ends: interest to each month/quarter/year end joins the principal from the 1st of the next month, i.e. 1 Jan/Apr/Jul/Oct for quarterly, or 1 Jan for yearly.
  • Daily: principal × ((1 + rate ÷ year days)^days − 1) within each period.
  • Continuous: principal × (e^(rate × days ÷ year days) − 1) within each period.

  Results compare the compounded interest with the same calculation as simple interest. On a compounding date, interest is added to principal first, then principal added later, then payments.

• Switch to a different rate from a date: e.g. prime + 1% before judgment, then the judgment rate after. The new rate (judgment, prime + spread, or fixed) applies from the switch date. Rows show each period's own rate, and the rates-used list labels each rate with its kind. Optionally, the interest accrued up to the switch date can be added to the principal (Interest before the change: Add to principal), so the new rate is charged on both; off by default.

• Currency: HKD by default, USD by default for the US prime rate; or RMB, the other G10 currencies (EUR, JPY, GBP, CHF, CAD, AUD, NZD, SEK, NOK) or any symbol you type. Amounts on the page and in every download use its symbol (HK$, US$, CN¥, €, JP¥, £, C$, A$, NZ$; codes for CHF, SEK and NOK). Once chosen, it doesn't change with the rate.

Cash flows: principal added later and payments received sit together in a collapsible Cash flows pane, which shows a count (e.g. "1 principal added, 2 payments") while closed and opens by itself for a link that has any.

Principal added later: further principal such as costs, each with a date, an amount and an optional description. From its date, each amount joins the principal and earns interest at the same rate. On a day with both, the principal is added before any payment, so the payment can clear it. Amounts dated outside the calculation period are ignored with a warning.

Icon and link preview: a calculator icon for browser tabs and the iPhone home screen, plus a preview card (title, description, image) when the link is shared. After changing site/favicon.svg or the preview wording, run npm run images.

Partial payments: add any number of payments (date + amount). Choose how they're applied:

• Interest first, then principal (default): each payment clears accrued unpaid interest, and the rest reduces the principal.
• Principal first, then interest.

Interest stays simple either way: unpaid interest never earns interest. A payment on a date counts from that day. Payments before the start date, or on or after the end date, are ignored with a warning, and any overpayment is flagged. The results show a payments table and the outstanding principal plus unpaid interest. The exports include these too; in Excel, each period's interest uses that period's principal as a live formula.

Daily interest thereafter: principal × the rate in force on the end date ÷ that day's year days. Under Actual/Actual it shows two figures, ÷ 365 for a normal year and ÷ 366 for a leap year, on the page and in every download. Use it for wording like "…plus HK$219.18 per day until payment". It appears on the page and in the Word, PDF and Excel files; in Excel it's a live formula.

Downloads (every tab with files): Download [Word | PDF | Excel] (Mortgage and PV: PDF | Excel), each a one-click download, with [Copy link | Save] beside it (below on a phone).

Download PDF: downloads a report straight away (built in the browser with jsPDF): inputs, results, the formula for each period, the rates used, and the sources as named, clickable links. No raw URLs are printed.

Sortable rate table: click Effective date or Rate to sort; click again to reverse. The PDF and Excel exports use the same order.

Shareable link: the inputs are stored in the page address (e.g. ?src=prime&p=1000000&from=2026-01-01&to=2026-09-30&basis=act%2Fact&round=total&spread=1). Copy link copies it.

Excel export: the Excel file uses live formulas and has a second sheet with the rate sources.

Word export: the interest schedule as a table worded like a statutory demand, one row per period: "(i) Interest on the sum of HK$… at the rate of …% per annum from … to … (n days)", the working ("(i.e. principal × rate × days ÷ 365 = amount)") and the amount, then the total, a summary (principal, any principal added or payments, interest, total amount due at the end date) and the daily interest from the end date until payment. Ready to paste into a court document.

Today's rates in the header: under "Rates updated as at…", the judgment debt rate, HSBC prime, US prime, and 1-month and 3-month HIBOR, each in the same style: name, rate in force today (3 decimals, or more when published with more, like HIBOR's 5) and the date it applies from.

Word calculation inputs: the Word file starts with a "Calculation inputs" block (rate, principal, dates, day count, rounding and any compounding or cash flows) above the table, so it explains itself when sent on.

Word source note: the Word file ends with a short note naming where the rates came from and when they were checked, e.g. "Source of rates: HK Judiciary: interest rates on judgment debts (as at 5 October 2026)." There's none for a fixed rate.

Recent calculations: the last 5 calculations you ran (any tab) are kept automatically in a "Recent calculations" list, in your browser only; open one, save it to keep it, or clear the list ("Clear recent calculations"; saved calculations stay). The sample run when the page opens isn't recorded.

Help: a "?" next to each Advanced setting, "Apply payments to" and the End date shows a short explanation; tap again to hide it.

Saved calculations: Save (on either tab) keeps the calculation's link and a name in your browser's local storage, never uploaded. The Saved calculations card lists them, newest first: rename, open or delete. Up to 50 are kept.

Rounding: Total rounds only the total (rows are added unrounded); Per row rounds each row of the table to cents first. The second option makes the rows add up exactly to the total.