Tools · Money Lab
Money Lab
Explore money. Test ideas. Understand the results.
Each experiment below shows one money idea in action. Change a number and the results, chart and takeaway update straight away. Nothing you type is saved or sent anywhere.
- 1Pick an experiment
- 2Change the numbers
- 3Read what it means
Experiment 1 of 3
Small Decisions, Big Impact
A small amount set aside every week doesn’t look like much. Over many years, and with growth building on growth, it can turn into a surprising sum. Pick a habit or type your own amount.
Investing vs. keeping it at 0%
What this shows
The return is a steady, made-up rate for illustration. Real investments rise and fall from year to year, can lose money, and nothing here is a forecast. Want dividends, inflation and a detailed table? Use Grow Investments on the Budget & Savings Calculator.
How this experiment works
Your weekly amount is turned into a monthly one: weekly × 52 ÷ 12. That money is added at the end of each month, and the yearly return is split into twelve equal monthly steps (rate ÷ 12) that apply to the whole balance. In other words, the rate is a nominal yearly rate compounded monthly, so 6% works out to about 6.17% over a full year.
Because each month’s growth is added to the balance, next month’s growth is calculated on a bigger number. That is compound growth: growth on top of earlier growth. It is small at first and becomes the larger part over long periods. New to the idea? Read the two-minute lesson on compound interest.
Fees, taxes and changes in the rate are left out to keep the idea clear.
See the numbers year by year
Experiment 2 of 3
The Inflation Experiment
Inflation means prices in general go up over time. Seen from your wallet, the same thing looks different: each dollar buys a little less. This experiment shows both sides at once, looking ahead or looking back.
Two views of the same change
What this shows
This uses one steady rate every year. Real inflation changes from year to year, and your own costs can rise faster or slower than the average. Pay often rises over time too, so this shows money that sits still, like cash in a drawer.
How this experiment works
Rising prices: future price = today’s amount × (1 + inflation)^years. At 3% for 10 years, $100 becomes about $134.
Falling buying power: buying power = today’s amount ÷ (1 + inflation)^years. At 3% for 10 years, $100 kept as cash buys about $74 worth of today’s goods.
These describe the same change from two sides, so the two lines in the chart move in opposite directions. The percentages differ (about 34% up versus 26% less at 3% for 10 years) because each is measured from a different starting point. Amounts measured in today’s prices are called real or inflation-adjusted; amounts in future dollars are called nominal.
See the numbers year by year
What was money worth in a past year? This side uses real U.S. prices from the Consumer Price Index, not an assumed rate.
Same buying power over the years
What this shows
Source: U.S. Bureau of Labor Statistics, Consumer Price Index for All Urban Consumers (CPI-U), U.S. city average, all items, not seasonally adjusted (1982–84 = 100). Past years use each year’s annual average, and “today” uses September 2026, the latest month published. 2025 isn’t offered because the October 2025 index was never published. Big jumps between neighboring years are real: prices rose fast around both World Wars and fell during the early 1920s and the Great Depression.
How this side works
value today = amount × (CPI today ÷ CPI in that year), and the reverse for what today’s money was worth back then. The average yearly rate is the steady rate that would give the same total rise over those years.
The CPI tracks the average price of a large basket of goods and services. Your own costs may have risen faster or slower: rent, college and medical care, for example, have often risen faster than the average. Comparisons far in the past are rougher, because what people buy has changed a lot since then.
See the numbers year by year
Experiment 3 of 3
The Cost of Waiting
Two friends save the same amount every month. Person A starts now. Person B waits a few years first. How much does waiting cost?
Change the time and return
– saved– growth
– saved– growth
Balances over time
What this shows
Both people use the same steady, made-up return. Real returns vary, can be negative, and the order of good and bad years also matters. This is an illustration, not a forecast.
How this experiment works
Both people add the same amount at the end of each month, and the yearly return is split into twelve monthly steps (rate ÷ 12), a nominal rate compounded monthly. Person A adds money every month of the period. Person B adds nothing during the waiting years, then the same amount every month until the same end date.
The gap has two parts. One part is simply the deposits Person B skipped while waiting. The other part is growth: those early dollars had the most years to grow, and their growth kept growing.
The bars show each person’s ending balance in two parts: the money they put in (solid) and the growth on it (lighter). With a negative return, the striped part shows money lost.
The catch-up figure is the monthly amount Person B would need, over the years left, to end with the same balance as Person A at the same assumed return.
See the numbers year by year
Words used on this page
- Return
- How much an investment gains or loses in a year, as a percentage. It can be negative.
- Compound growth
- Growth that is added to your balance and then grows too, so the increases get larger over time.
- Inflation
- The general rise in prices over time.
- Purchasing power (buying power)
- How much a set amount of money can actually buy.
- Nominal vs. inflation-adjusted
- Nominal is the dollar figure you would see at the time. Inflation-adjusted (or real) shows it in today’s prices.
Money Lab is for education only and is not financial, investment or tax advice. Every rate on this page is an assumption you choose; it is not a prediction or a record of past performance. Investing involves risk, including losing money. Results leave out fees, taxes and changing rates. Consider speaking with a qualified professional about your own situation.