About
About avgr
The average down calculator built on real math
What avgr does
avgr (avgr.app) is a free, browser-based average down calculator for retail investors. You enter three numbers — shares owned, your original buy price, and the current market price — and avgr instantly shows you:
- Your new average cost per share at any capital amount you choose
- Your break-even price — how much the stock needs to recover for your position to turn profitable
- The full diminishing-returns curve — a visual showing exactly how much each additional dollar moves your average, and where the improvement flattens out
- Capital efficiency — how much your average price improves per $100 deployed at each point on the curve
avgr works with any asset priced per unit: stocks, ETFs, index funds, REITs, crypto, or anything else you can express as shares owned plus a price. No ticker lookup, no account required — just the numbers.
Why avgr exists
Most averaging-down decisions get made with a rough mental model: "if I buy X more shares, my average comes down a bit." That model almost always understates how little capital actually moves the needle — and how quickly the improvement flattens.
The law of diminishing returns in cost averaging is real and mathematically certain. Your existing position anchors your average cost. As you add more shares, each new purchase represents a smaller and smaller fraction of your total holding, so its influence on the weighted mean shrinks. There is a hard ceiling — the current market price — that no amount of capital can cross.
avgr was built to make that curve visible, so investors can see exactly what they're buying before they commit capital. That's it. No recommendations, no signals, no predictions — just the math, done precisely, so you can decide with clear information.
The math behind avgr
The core calculation is a weighted average:
Where Q0 is shares currently held, P0 is your original average buy price, and shares_bought is floored to the nearest whole unit (discrete share purchases). The improvement — delta — is P0 − P_new. The curve is plotted across 121 evenly spaced capital points, then interpolated smoothly using monotone cubic Hermite (Fritsch-Carlson) interpolation.
All arithmetic uses 28-digit decimal precision via Elixir's Decimal library, avoiding the floating-point rounding errors that affect spreadsheets and most online calculators. When you enter $180.00 and buy 20 shares at $155.00, you get the mathematically correct result — not a float approximation.
The slider upper bound (sliderMax) is set to the capital needed to reach 90% of the theoretical maximum improvement — the point beyond which you are firmly in the flat tail of the curve. This keeps the slider focused on the decision-relevant range without imposing an arbitrary cap.
Who built avgr
avgr is a solo project, built and maintained by a software developer who also invests. It started as a personal tool — a quick way to answer "how much would I actually need to buy to move my average down meaningfully?" — and grew into a public product when it became clear no existing calculator visualised the diminishing-returns curve clearly.
The site has no investors, no ads-driven growth targets, and no data to sell. It is free, it will stay free, and the only goal is to be the most accurate, honest averaging-down calculator available.
Questions or feedback? Get in touch.
Not financial advice
avgr is a mathematical tool. It computes arithmetic relationships between numbers you enter. It does not recommend whether you should buy, sell, or hold any security. A lower average cost per share does not guarantee profitability — the stock can keep falling regardless of how low your cost basis is. Always consult a qualified, licensed financial adviser before making investment decisions.