Shares calculator: how many shares to buy

Last updated: August 22, 2026.

To find how many shares to buy, multiply your contribution by each position’s target percentage, then divide by that position’s share price: shares = (contribution × target %) ÷ share price. The arithmetic is easy. The awkward part is that share prices rarely divide evenly into your contribution, so buying whole shares always leaves some cash uninvested — and the smaller your contribution, the bigger that leftover is as a share of the total.

The formula

For each position you hold:

1. Dollar target = contribution × target percentage
2. Exact shares = dollar target ÷ current share price
3. Shares to buy = the exact figure, rounded down to a whole share (unless your broker supports fractional shares)

Rounding down rather than to the nearest whole share matters: rounding up means committing money you have not deposited, and the order is rejected or settles into a cash debit.

Worked example: a $6,000 contribution across three funds

Target allocation of 70% US stocks, 20% international, 10% bonds. Share prices here are illustrative round numbers for the arithmetic, not quotes:

You buy 15, 19 and 8 shares, spend $5,777.39, and $222.61 stays in cash — 3.7% of the contribution.

What the rounding actually costs you

The intuitive worry is that rounding wrecks your allocation. It mostly does not. Measured against what you actually invested, the example above lands at:

Three tenths of a point is noise. The real cost is the uninvested $222.61, not the mix. That reframes the problem: you are not trying to protect your allocation from rounding, you are trying to stop cash piling up in the account. Which is why the honest options are to carry the remainder into next month’s contribution, or to top up the position that is furthest below its dollar target if a single share fits inside the leftover.

In the example, no single US share fits — one costs $268.42 and only $222.61 is left — so the disciplined move is to carry it forward rather than distort the plan buying something you wanted less of.

Small contributions are the hard case

The same three funds and the same 70/20/10 target, but a $500 monthly contribution:

Now $170.23 is left over — 34% of the contribution — and the bond sleeve gets nothing at all, because half a target dollar amount of $50 does not reach one $73.18 share. Repeat that monthly and the bond allocation quietly stays at zero while cash accumulates.

This is the single best argument for either fractional shares or a deliberate rotation, described below. It is also why "just divide it up" advice falls apart in real accounts: the maths is fine, the share prices are the obstacle.

Whole shares versus fractional shares

If your broker supports fractional shares for the funds you hold, most of this problem disappears — use the exact figure from step 2 and stop there. If it does not, plan around the rounding instead of fighting it.

The rotation approach

Rather than underfunding the same position every month, rotate which position absorbs the leftover. In the $500 example, the bond sleeve gets nothing in month one; if you carry the $170.23 forward, month two has $670.23 to work with and one $73.18 bond share comfortably fits. Over a few months the average allocation tracks the target closely even though no single month does.

Using new contributions to rebalance

If a position has drifted above its target, the reflex is to sell some. In a taxable account that realises gains; in any account it costs a round trip of spreads. Directing new contributions toward the underweight positions moves you back toward target without selling anything.

The arithmetic changes slightly: instead of applying target percentages to the contribution, apply them to total portfolio value after the contribution, then buy the difference.

1. Post-contribution total = current portfolio value + contribution
2. Position target = post-contribution total × target %
3. Dollars to add = position target − current position value (skip anything already at or above target)
4. Shares to buy = dollars to add ÷ share price, rounded down

With a large enough portfolio relative to the contribution, this will sometimes tell you to put the entire contribution into one position. That is correct, not a mistake.

Common mistakes

• Rounding to the nearest share instead of down. Rounds up commit money you have not deposited.
• Using yesterday’s price. The share count is only valid at the price you used. A few percent of price movement changes the answer, most visibly on high-priced shares.
• Applying target percentages to the contribution when you meant to rebalance. Those are two different calculations, and the second one is the one that fixes drift.
• Ignoring the leftover. A few hundred dollars a month sitting as cash is a real drag; it needs a home, even if that home is next month.
• Forgetting the contribution cap. If this is an IRA or HSA, the annual limit binds regardless of the share maths. See the 2026 contribution limits and Roth IRA limits.