Free compound interest calculator — see how steady saving and Solomon's wisdom on diligence (Proverbs 13:11) build long-term stewardship wealth.
The compound interest formula
The classic equation is A = P(1 + r/n) nt :
- A — future value
- P — principal (starting balance)
- r — annual interest rate as a decimal (8% → 0.08)
- n — number of compounding periods per year
- t — number of years
When you add monthly contributions, you also add the future value of an annuity: PMT × ((1 + r/12) 12t − 1) / (r/12). This calculator runs both equations together.
Why compound interest is the eighth wonder of the world
Albert Einstein is widely quoted as calling compound interest "the eighth wonder of the world. He who understands it, earns it; he who doesn't, pays it." Whether or not he actually said those exact words, the math is undeniable: $500/month invested at 8% becomes about $745,000 in 30 years — even though you only contributed $180,000 of your own money. The market did the other $565,000 of work.
Time is the rocket fuel. The same $500/month for only 20 years grows to roughly $295,000. Cut to 10 years and you're at $91,000. Doubling the years more than doubles the money — that asymmetry is why Christians who start early steward radically more than those who wait.
Is compound interest biblical?
Yes — patient, productive accumulation is repeatedly commended in Scripture. Proverbs 13:11: "Wealth gained hastily will dwindle, but whoever gathers little by little will increase it." This is literally a description of compounding. Proverbs 21:5: "The plans of the diligent lead surely to abundance, but everyone who is hasty comes only to poverty."
The parable of the talents (Matthew 25:14-30) explicitly rebukes the servant who buried his master's money instead of putting it to work — even bank interest would have pleased the master ("you ought to have invested my money with the bankers, and at my coming I should have received what was my own with interest" — v. 27). Investing wisely is not the opposite of trusting God; it is part of faithful stewardship.
The ethical line in Scripture is clear though: Exodus 22:25 and Leviticus 25:35-37 forbid charging interest to the poor among God's people. Predatory lending and usury are condemned. Compound growth on your own honest savings is not.
Realistic rate of return
- High-yield savings: 4–5%
- Bonds (long-term avg): 4–6%
- S&P 500 (long-term avg, 1928–today): ~10% nominal, ~7% after inflation
- Total stock market (Bogleheads default): ~7% real return for planning
For long-horizon planning (retirement, kids' college, generational wealth), 7–8% is the conservative number most planners use. Don't model 12% — that's a sales-pitch return, not a math return.
Related calculators & guides
- Emergency Fund Calculator — fund this first, then invest
- Debt Snowball Calculator — kill high-interest debt before chasing returns
- Net Worth Calculator — track what you've stewarded
- 50/30/20 Budget Calculator — find the monthly contribution
- The parable of the talents — full meaning
- 10 biblical money management principles
Frequently asked questions
Is investing biblical?
Yes. The parable of the talents (Matthew 25:14-30) explicitly commends putting money to work and rebukes the servant who buried his. Proverbs 13:11 says wealth gathered 'little by little' grows. Scripture warns against get-rich-quick schemes (Proverbs 28:20), not against patient compounding.
What is the Rule of 72?
Divide 72 by your annual return rate to estimate how many years it takes money to double. At 8%, money doubles every 9 years. At 10%, every 7.2 years. Use this to project the long-horizon stewardship Jesus calls 'faithful with little' (Luke 16:10).
Should Christians invest in index funds?
Scripture is silent on instrument. Most Christian planners recommend low-cost, broadly diversified index funds because they match Ecclesiastes 11:2 ('divide your portion to seven, even to eight') — diversification as protection. Avoid funds tied to industries that violate your conscience.
How much should I invest each month?
Most Christian financial teachers recommend 15% of gross income to retirement once the emergency fund is full and consumer debt is gone. The Solomon Wealth Code app shows you exactly how that 15% compounds across 10, 20, and 30-year horizons.