Assumption: monthly compounding. Each month applies growth first, then contribution, then withdrawal (if drawdown is active).
How this compound interest calculator with drawdown works
Most basic investment calculators only answer one question: “What happens if I keep adding money?” Real life adds a second phase: “What happens when I start taking money out?” This page models both. You can simulate an accumulation phase, a drawdown phase, or both together.
In every monthly step, the model does three things in order: (1) applies investment return, (2) adds your monthly contribution, and (3) subtracts your monthly withdrawal once drawdown starts. That order is clearly shown so your assumptions stay transparent.
Input fields explained
Starting Balance
The amount currently invested. This becomes your base principal and is included in total contributed capital.
Annual Return Rate
The expected average yearly return before taxes and fees. The calculator converts this to a monthly rate for compounding. You can test optimistic and conservative scenarios by running multiple rates (for example: 4%, 6%, 8%).
Years to Simulate
Your full timeline. If you are 35 and planning to review your portfolio through age 70, you might model 35 years.
Monthly Contribution
New money added every month. Use this to represent regular investing from salary, business cash flow, or side income.
Drawdown Start Year and Monthly Drawdown
This pair controls withdrawals. Set Drawdown Start Year to 0 if you want growth-only simulation. Otherwise, choose the year when withdrawals begin and how much you intend to take each month.
Annual Drawdown Increase
Withdrawals often rise over time due to inflation or lifestyle changes. This setting raises annual withdrawals by a percentage each year after drawdown begins.
Why drawdown modeling matters
Accumulation can hide risk because balances generally trend upward over long periods. Drawdown introduces a fragile period: if withdrawals are too high, or market returns are weak early in retirement, the portfolio may struggle to recover. This is often called sequence of returns risk.
- Two portfolios can have the same average return but very different outcomes in drawdown.
- Early poor returns combined with fixed withdrawals can shrink principal quickly.
- A flexible spending strategy can materially improve longevity of the portfolio.
How to interpret results
After calculation, you get key metrics:
- Ending Balance: how much remains at the end of the timeline.
- Total Contributed: starting principal + all monthly deposits.
- Total Withdrawn: cumulative cash taken out during drawdown.
- Net Growth: ending balance + withdrawals − contributions.
The yearly schedule helps you spot trends: when withdrawals overtake growth, whether balances flatten, and whether your plan remains sustainable.
Practical planning tips
Run multiple scenarios, not one
Don’t rely on a single expected return. Compare conservative, base, and optimistic assumptions.
Stress-test withdrawal amounts
Try higher drawdown figures and inflation adjustments to see where your plan begins to fail. This helps define a safer spending range.
Review annually
Real markets don’t follow perfect averages. Revisit your assumptions once per year and adjust contributions or withdrawals as needed.
Limitations to remember
This calculator is intentionally simple and educational. It does not include taxes, fees, account types, changing asset allocation, or random market volatility. Use it to understand direction and sensitivity, then combine it with detailed planning if you are making major decisions.