General rules
Every calculator runs on our server with exact decimal arithmetic, so the same inputs always give the same result, with or without JavaScript. Amounts are shown rounded to two decimal places and percentages to two decimal places; rounding only happens when a value is displayed, never between the steps of a calculation.
The currency you choose on a calculator is only the unit of the amounts. No exchange rate is applied and nothing is converted: the arithmetic is the same in every currency. If you do not choose one, pages in English start with US dollars.
Unless a page says otherwise, results do not include taxes, account or fund fees, spreads or inflation. They are gross, nominal figures.
Stock average calculator
Buys and sells are processed in the order you enter them. Each buy adds its quantity to the position and its cost, including the buy fee, to the total cost:
Average cost = total cost of the shares you hold ÷ number of shares you hold
A sale removes shares at the current average cost. The difference between the sale proceeds (after the sell fee) and that cost is the realized profit or loss; the average cost of the shares you keep does not change. A sale cannot exceed the shares held at that point. If you enter a current price, the unrealized profit or loss of the remaining shares is calculated against the same average cost. The calculator uses the same rule as the transaction ledger in the Numfolio app.
Compound interest calculator
The balance is multiplied by (1 + annual rate ÷ n) in every compounding period, where n is the number of periods per year (1, 2, 4, 12 or 365). A regular contribution is added at the end of each period and earns interest from the following period on.
Balance = principal × (1 + annual rate ÷ n)n × years
Effective annual rate = (1 + annual rate ÷ n)n − 1
The calculation is carried out period by period, so the yearly table and the final balance always agree. Very large inputs that would overflow the calculation are rejected with a message instead of producing a wrong number.
ROI calculator
The amount returned is the final value plus income received minus costs paid. ROI is the net profit (amount returned minus amount invested) divided by the amount invested. If you enter a holding period, the annualized return is (amount returned ÷ amount invested)1 ÷ years − 1. Fractional powers are computed in decimal arithmetic through the natural logarithm and exponential series (about 25 significant digits) and shown rounded to two decimals. If the amount returned is negative, no annualized rate is shown.
CAGR calculator
CAGR = (ending value ÷ starting value)1 ÷ years − 1, with the number of years allowed to include fractions. An ending value of zero gives −100%. For whole periods of up to 50 years, a table shows the value at the end of each year at the calculated rate; the last row is the ending value you entered.
Stock profit calculator
Total cost is shares × buy price plus the buy fee; net proceeds are shares × sell price minus the sell fee. Profit before tax is the difference. The optional tax rate is applied to a profit only, as a simple percentage; a loss is never taxed. The return on cost is the net profit divided by the total cost, and the break-even sell price is (shares × buy price + both fees) ÷ shares.
Average down calculator
The new average is (shares held × current average + new shares × new price) ÷ total shares, the same rule as the stock average calculator without fees. The shares needed for a target average are shares held × (current average − target) ÷ (target − new price); a target outside the range between the new price and the current average is rejected. The table repeats the calculation for 0.5, 1, 2 and 3 times the planned quantity.
Entering numbers
On English pages the dot is the decimal separator ("1.5") and the comma groups thousands ("1,250,000"). A number with commas in the wrong places is rejected rather than guessed. Pages in other languages follow their own conventions, and the server always reads the number in the language of the page.
Reviews and updates
Each page shows the day its content last changed. When we only re-check a page without changing it, the review date is shown separately. The worked examples in the articles are checked automatically against the calculator, so a change to a rule also changes the text that describes it.
Limits
- The calculators are for information only; they are not an offer to buy or sell and not investment, tax or legal advice.
- Results depend entirely on the figures you enter. Check important numbers against your broker or bank statements.
- Tax rules, fees and inflation differ between countries and products and are not modelled unless a page says so.
