Historical DCA Backtester

This backtester simulates what would have happened if you invested a fixed dollar amount on the first trading day of every month over your chosen time period. Here's the exact methodology:

Step-by-step calculation:

• Monthly purchase simulation — On the first available trading day of each month, we divide your monthly investment by the stock's split-adjusted closing price to calculate the number of shares purchased.
• Cumulative tracking — We keep a running total of shares owned and dollars invested. The portfolio value at any point equals total shares times the stock price on that date.
• Split-adjusted prices — We use split-adjusted close prices (adjClose) so that stock splits and dividends are properly accounted for in the historical data.
• Final valuation — Your current portfolio value is calculated by multiplying total shares accumulated by the most recent closing price.

Key metrics explained:

• Total Return (%) — The absolute percentage gain or loss: (Current Value - Total Invested) / Total Invested.
• CAGR — Compound Annual Growth Rate, which annualizes your total return to show what equivalent annual rate would produce the same result.
• Average Cost Basis — Your total dollars invested divided by total shares, representing your break-even price per share.

This is one of the most debated questions in personal finance. Academic research, including studies by Vanguard and others, generally shows that lump-sum investing outperforms DCA about two-thirds of the time, because markets tend to go up over time — so the sooner you're fully invested, the more time your money has to grow.

However, DCA wins in other important ways:

• Behavioral advantage — Most investors don't have a large lump sum sitting around. DCA matches how people actually earn and save money (monthly paychecks).
• Risk reduction — DCA significantly reduces the risk of investing your entire sum right before a major drawdown. The emotional damage of a 30% loss on day one can cause investors to sell at the worst time.
• Consistency beats perfection — An investor who DCA's religiously for 20 years will almost certainly outperform someone who waits for the "perfect" entry point and misses months or years of gains.

The bottom line: if you have a lump sum and a long time horizon, lump-sum investing has a slight statistical edge. But if you're investing from income over time, DCA is not just good — it's the natural and effective approach.

The time period you choose dramatically affects your results, and understanding why helps you interpret the numbers correctly.

Period recommendations:

• 1 Year — Too short for meaningful DCA analysis. Useful for seeing recent momentum, but a single year can be dominated by one event (earnings surprise, macro shock). Don't draw long-term conclusions from 12 months.
• 3 Years — A reasonable minimum. Captures at least one market cycle and shows whether DCA smoothed out volatility. Good for evaluating recent performance trends.
• 5 Years (recommended) — The sweet spot for most investors. Long enough to include bull and bear markets, short enough that the company's fundamentals are still relevant. Most financial advisors use 5-year returns as a benchmark.
• 10 Years — Best for seeing the true power of compounding and DCA. Includes multiple market cycles and shows how consistency pays off over a decade. However, a company can change significantly over 10 years, so the backtest may not reflect the company's current trajectory.

A useful exercise is to run the same ticker across multiple periods to see how returns change. If a stock looks great over 1 year but poor over 5 years, that recent performance might be a dead-cat bounce rather than a sustainable trend.

CAGR (Compound Annual Growth Rate) is the annualized rate of return that takes compounding into account. It answers the question: "What steady annual return would have produced the same final result?"

For DCA specifically, CAGR is calculated differently than for a lump-sum investment because money is invested at different times. Our calculation uses the total invested amount and the final portfolio value spread over the investment period.

Benchmark CAGRs to compare against:

• S&P 500 historical average — Roughly 10% annually (including dividends) since 1926, or about 7% after inflation.
• Risk-free rate — Treasury bills yield around 4-5% in the current environment. Your CAGR should exceed this to justify the risk of owning stocks.
• Inflation — Running around 2-3% annually. Subtract this from your CAGR to get your real (inflation-adjusted) return.

A CAGR above 12-15% over 5+ years is considered excellent. Below 5% over the same period, and you may have been better off in a savings account. But remember: CAGR is backward-looking. Past performance doesn't guarantee future returns — which is exactly why combining DCA backtesting with forward-looking valuation (like a DCF model) gives you the full picture.

Your average cost basis represents the average price you paid per share across all your monthly purchases. It's calculated by dividing your total dollars invested by your total shares accumulated. This number will almost always differ from the current stock price — and the gap between them tells an important story.

If your cost basis is below the current price: You're in profit. DCA worked in your favor by averaging your purchases across lower prices in the past. The bigger the gap, the more your investment has grown.

If your cost basis is above the current price: You're currently at a loss. This can happen when a stock has declined significantly from its historical levels. However, if you continue DCA'ing, you'll be buying at these lower prices, which pulls your cost basis down over time.

This is why DCA advocates say "a falling stock price is your friend when you're still accumulating." You get more shares for the same dollar amount, lowering your break-even point. The risk, of course, is if the stock never recovers — which is why DCA works best with diversified index funds or fundamentally strong companies.

Stock splits: Yes. We use split-adjusted closing prices (adjusted close) from our data provider, which retroactively adjusts all historical prices for stock splits. This means a stock that traded at $400 pre-split and $100 post-split (4:1 split) will show the pre-split price as $100 in the data, giving you an accurate picture of returns.

Dividends: Partially. The adjusted close price accounts for the impact of dividends on the stock price (specifically, the price drop on ex-dividend dates is adjusted out). However, this backtester does not simulate dividend reinvestment (DRIP). In reality, if you reinvested dividends by buying additional shares, your returns would be somewhat higher than shown here.

What's not included:

• Transaction costs — Most brokers now offer commission-free trading, so this is negligible for most investors.
• Taxes — Capital gains taxes on selling are not modeled. Your actual returns after taxes will be lower.
• Fractional shares — We assume you can buy fractional shares (which most modern brokers support). If your broker doesn't, actual results may vary slightly.

A backtest shows you what happened in the past, but the real value is in how you use that information to make forward-looking decisions. Here's a framework:

1. Evaluate the consistency of returns: Run the same ticker over multiple periods (1Y, 3Y, 5Y, 10Y). A stock with strong returns across all periods is more reliable than one that only shines over 1 year.

2. Compare against benchmarks: Is the stock beating the S&P 500 over the same period? If you're taking on single-stock risk but not outperforming the index, you might be better off DCA'ing into an ETF like SPY or VOO.

3. Look at the chart shape: A portfolio value line that consistently tracks above the cash invested line suggests strong momentum. If the two lines cross multiple times, the stock has been volatile and your timing matters more.

4. Pair with fundamental analysis: Past performance doesn't guarantee future results. A backtest tells you where the stock has been — a DCF (discounted cash flow) model tells you where it's going. The backtest is your rearview mirror; the DCF is your windshield.

The ideal workflow: run a DCA backtest to see historical performance, then build a DCF model to determine if the stock is fairly valued today. If DCA returns have been strong AND the stock is undervalued, you have a compelling case to continue or increase your position.