Portfolio Drift Explained: How to Measure It

What Drift Actually Is

You set a target: 60% equities, 40% bonds. Time passes. Equities have a good run, bonds do not. Your money is now split 66/34.

Nothing went wrong. You did not make a mistake. The allocation moved because that is what allocations do when their components perform differently. The only real problem is that the portfolio is now carrying a risk level you did not choose, and if you never look, you will not know by how much.

Percentage Points, Not Percent

Worth fixing this before the arithmetic, because most articles are sloppy about it and the two readings differ by a lot.

If your target is 60% and you are now at 66%, you can describe that two ways:

• 6 percentage points over. The difference between the two percentages.
• 10% over. Six is ten percent of sixty.

These are different statements. "6% over a 60% target" would mean 63.6%, not 66%. The percentage-point version is the one worth using, because it maps straight onto an action: being 6 points overweight means about 6% of the portfolio's value needs to move to close the gap. The relative version needs a second calculation before it tells you anything.

How to Compute It

Two steps: find your current weights, then subtract the targets.

A Worked Example

You started with EUR 100,000 split 60/40. Over the period, equities returned 30% and bonds were flat.

Equities: EUR 60,000 x 1.30 = EUR 78,000
Bonds:    EUR 40,000 (unchanged)
Total:    EUR 118,000

Now the weights:

Equities: 78,000 / 118,000 = 66.1%
Bonds:    40,000 / 118,000 = 33.9%

And the drift:

Equities: 66.1 - 60 = +6.1 points (overweight)
Bonds:    33.9 - 40 = -6.1 points (underweight)

Notice the two are mirror images. In a two-slice portfolio they always are, because the weights must add to 100. With more slices they do not cancel so neatly, which is where the next point comes in.

Two Ways to Summarise It

Once you have more than two holdings, "how far have I drifted" needs a definition:

• Largest single deviation. The worst offender. Simple, and it is what a threshold rule usually watches.
• Total absolute deviation. Add up every slice's drift ignoring the signs. Captures a portfolio that has crept in many small ways rather than one large one.

They can disagree completely. A portfolio where one holding is 7 points over and everything else is on target looks bad on the first measure and mild on the second. Reversed, a portfolio where a dozen holdings are each 1 point out looks calm on the first and messy on the second. Pick one and be consistent, or the number will seem to jump around for no reason.

The Currency Complication

Here is the part that catches European investors, and almost nothing written about drift mentions it.

Your allocation can move when no asset moves at all.

Suppose your EUR 60,000 of equities is half European and half US-listed, and your EUR 40,000 of bonds is all in euros. The dollar strengthens 10% against the euro. Every share price stays exactly where it was.

US equities:  EUR 30,000 x 1.10 = EUR 33,000
EU equities:  EUR 30,000 (unchanged)
Equities:     EUR 63,000
Bonds:        EUR 40,000
Total:        EUR 103,000

Equity weight: 63,000 / 103,000 = 61.2%
Drift:         61.2 - 60 = +1.2 points

You are 1.2 points overweight equities and not a single holding changed price. The FX move did it.

This matters because it means your equity exposure and your currency exposure are not independent. A euro investor holding US assets is making two bets, and drift will register the second one whether or not they meant to make it. Our article on currency gain separates the two effects on the return side; drift is the same split showing up on the allocation side.

The Multi-Broker Complication

Drift is a property of the whole portfolio, not of any account within it.

If your equities sit at one broker and your bonds at another, neither broker's app can tell you your allocation. Each shows you 100% of itself. You can be 8 points overweight equities across your holdings while both screens look perfectly reasonable.

This is the ordinary situation, not an edge case: people accumulate brokers through moving country, changing employer, or chasing a better fee on one asset type. Any drift figure computed inside one account is answering a question you did not ask. See consolidating brokerage accounts for the general version of the problem.

How Much Drift Is Worth Noticing

There is no universal answer, and anyone offering one is guessing about your situation. What exists is common practice and some concrete reference points.

The most widely cited convention is a 5 percentage point band: act when a slice is more than 5 points from target, ignore it otherwise. It appears in Vanguard's rebalancing research and in most practitioner writing.

For a concrete reference at the individual-holding level, Turbobulls keeps a gap under one point grey and colours it past that: orange when a holding is over its target, blue when it is under. Neither is red, deliberately, because being off target between rebalances is normal rather than an error. That is the product's threshold rather than a recommendation, but it gives you a sense of the scale at which drift stops being rounding.

What to Do About It

Briefly, because it has its own article: you can sell what has grown and buy what has not, or you can direct new contributions toward the underweight side and let the portfolio correct itself without any sale. The second is usually cheaper and does not realize a gain.

Which schedule to follow, what a correction actually costs, and the order to do it in are all covered in how to rebalance your portfolio.

How Turbobulls Shows Drift

Each holding carries a Weight, its market value as a share of your portfolio's, and a target weight you set shows as a mark on that weight's bar. The "Against your targets" card in Allocation lists every holding with a target and the difference between the two as drift in percentage points, positive meaning overweight.

Against your targets

5 holdings have a target, 71% of the portfolio together. You have €6,400 cash at your brokers.

Investing this month?

HoldingNow, and the targetNowTargetOff byBuy

VWRL Vanguard FTSE All-World46.2%50.0%-3.8 pts€308about 2.54 shares46.2% of 50.0%Buy €308

SXR8 iShares Core S&P 5007.2%10.0%-2.8 pts€201about 0.27 shares7.2% of 10.0%Buy €201

MSFT Microsoft3.2%4.0%-0.8 pts€58about 0.15 shares3.2% of 4.0%Buy €58

BTC Bitcoin3.6%4.0%-0.4 pts€33about 0.00036 BTC3.6% of 4.0%Buy €33

TSLA Tesla3.5%3.0%+0.5 pts3.5% of 3.0%+0.5 pts

The mark on each bar is the target. Click a target to change it. The money is split by how far each holding is below its target, and no buy goes past one. It is a suggestion.

Two details worth knowing, because they are the kind of thing that makes a number trustworthy or useless:

• Read drift with no filter on. Your targets are shares of the whole portfolio, but a filter narrows every figure to what it keeps, weights included: filter down to two holdings and their weights add up to 100 on their own. The table's search box only hides rows, so it leaves the weights as they are.
• Broker cash is excluded by construction, so the weights add to 100 rather than stopping at 87 with nothing on screen explaining the rest.

Targets and drift appear only for positions you still hold, so a sold-out row does not carry a target it can never meet.

Alongside this, allocation breakdowns by asset type, broker, currency and tag are on every plan. Breakdowns by sector, industry, country and region are on a paid plan.

The Full Picture: Read These Next

Frequently Asked Questions

Q: Is some drift fine?

Q: Does cash count?

Q: Does drift mean I picked the wrong allocation?

Turbobulls is a tracking and analytics tool, not an investment adviser. Nothing here is investment, tax, or legal advice. Investing involves risk, including loss of principal. Do your own research or consult a licensed professional.