Why VWAP differs from the price you paid

How volume changes the market average#

For a trade-based calculation, multiply each trade's price by its share quantity, add those amounts, then divide by the total shares traded. Larger trades carry more weight. The SEC's description of volume-weighted average price sets out this arithmetic.

Imagine a hypothetical market record containing these two trades:

Divide $11,800 by 1,000 shares and VWAP is $11.80.

Simply averaging the two prices would give $11.00. That treats a 100-share trade as equally important as a 900-share trade, even though most shares changed hands at $12.

Calculate your own purchase average#

Suppose your fills during that period were 20 shares at $10 and another 20 at $12, included within those market trades.

Your average execution price is $440 divided by 40 shares, or $11.00, before fees. Both figures can be correct: $11.80 for the market record and $11.00 for your purchases.

The calculation uses the same weighting method. It includes a different set of quantities. If you had bought 20 shares at $10 and 80 at $12, you would have spent $1,160 for 100 shares. Your average would be $11.60. The larger $12 purchase pulls the average towards $12.

Keep the execution average separate from total cash spent. In the original 40-share example, a $4 dealing fee takes the cash outlay from $440 to $444, or $11.10 per share. The trade prices still average $11.00. Your broker's tax cost-basis figure can include fees and other adjustments under the applicable rules; it need not equal the execution average. Use your fill confirmations and fee entries to reconstruct the comparison. FINRA on US cost basis

Match the time window#

A session VWAP builds as trading takes place. A full-day figure includes trades that occurred after an early purchase.

Suppose the 100-share market trade at $10 occurred at 10am and the 900-share trade at $12 occurred at 2pm. After the first trade, the record's VWAP was $10. After the second, it was $11.80. A purchase at $10 looks $1.80 below the later benchmark, but the 2pm trade had not happened at 10am. That comparison does not show that $11.80 was available when you made the decision.

An anchored VWAP starts the calculation at a chosen time, then includes subsequent trading up to the point displayed. Starting at this morning's open, the beginning of the month or a company announcement includes different trades and quantities. A monthly benchmark and one morning's fills answer different questions.

Write down the benchmark's start and end before comparing it with your trade.

Check how the chart calculates VWAP#

A chart can use one representative price for each time bar rather than every individual trade. Consider a separate hypothetical minute containing these trades in order:

The trade-based VWAP is $11,700 ÷ 1,000 = $11.70. If a bar-based method instead uses the average of the high, low and closing price, it takes ($12 + $10 + $11) ÷ 3 = $11 as the minute's representative price. For this one-bar example, its volume-weighted result is therefore $11.00.

The $0.70 difference comes from replacing all the trade prices with one bar price. Both calculations use the same 1,000-share volume, but the bar's high, low and close do not show that 800 shares traded at $12. This is an illustration of that method, not a claim that every chart uses it. Check your chart's price source, bar interval and anchor before comparing its result with your fills.

For a useful execution comparison, use a benchmark that matches the trading window and market data you intend to evaluate. Otherwise, the gap can partly reflect the setup of the indicator.

What paying below VWAP tells you#

In the example, you paid $0.80 less per share than the market's $11.80 volume-weighted average. Across your 40 shares, that is a $32 execution-price difference relative to that benchmark.

It is not $32 of investment profit. If the stock later trades at $9, your 40 shares are worth $360 against the $440 spent: an $80 loss before fees.

VWAP describes where the included trading happened. It does not estimate the cash a company will generate for shareholders or account for debt that must be repaid. A purchase can beat that day's trading average and still lose money because the business weakens or the price paid assumes growth that does not arrive. Record the execution comparison separately from your business and valuation assessment.