How to Calculate Portfolio Drift (and Rebalance Without Spreadsheets)

Alignfolio was built around one practical question:

The investment industry provides extensive guidance on when portfolios should be rebalanced, but much less guidance on how a limited cash contribution should be divided across several underweight asset classes. Alignfolio documents its methodology openly so the calculation is transparent and repeatable rather than a black box.

There is broad agreement across the investment industry on a few core principles: monitoring allocation drift, using new cash before selling whenever possible, and avoiding unnecessary trades.

While there is broad agreement on the principles of portfolio rebalancing, we are not aware of a universally accepted published methodology describing how a limited cash contribution should be allocated across multiple simultaneously underweight asset classes.

Alignfolio openly documents every calculation used to produce its recommendations. Nothing is hidden behind a proprietary score or black-box algorithm. Every recommendation can be reproduced manually using the methodology described on this page. The goal isn’t to claim a single mathematically “correct” formula. The goal is to produce recommendations that are:

• practical for day-to-day investing,
• explainable,
• consistent from one calculation to the next,
• and aligned with the investor’s stated target allocation.

If you use a different methodology yourself - including one based on relative drift - the principles above still apply. Alignfolio’s specific calculation is a documented design choice, not an industry standard.

Different investors may reasonably prefer different methodologies. Alignfolio’s goal is not to eliminate every valid approach, but to apply one documented methodology consistently so every recommendation is transparent, reproducible, and easy to explain.

Portfolio drift is the difference between your target asset allocation and your actual allocation.

• Target: 60% stocks / 40% bonds
• Actual: 70% stocks / 30% bonds

You didn’t decide to take more risk. It just happened.

Markets move. Some assets outperform others. Over time, your portfolio drifts away from your original plan.

This isn’t just a technical detail. It changes your risk - sometimes a lot.

Let’s say you have a $100,000 portfolio:

You’re now taking significantly more equity risk than you planned.

If your 60/40 drifts to 75/25 during a strong market, it feels great... until it doesn’t.

You’re not just “winning” - you’re now taking more downside risk when the market corrects.

Most investors only notice this after the drop, not before it.

You don’t need anything complicated. Just three numbers:

This works fine with 2 assets.

But what if you have:

• multiple ETFs
• ongoing contributions
• dividends
• different accounts

Now you’re doing:

• multiple calculations
• constant updates
• fixing broken spreadsheets

This is where most people give up.

A lot of investors try to solve this with Excel.

It works... until it doesn’t.

• formulas break
• adding new cash throws everything off
• updates get skipped
• it becomes too annoying

So what happens?

You stop tracking drift.

And your portfolio quietly moves further away from your plan.

Instead of rebuilding calculations every time, you just need:

• your current portfolio values
• your target allocation

From there, the only thing that matters is how far off you are - in % and dollars.

That’s it.

Most tools want you to connect your accounts and sync everything automatically.

That sounds convenient - but it also means giving access to your financial data and relying on syncing.

Alignfolio takes a different approach.

You enter your numbers manually.

It takes a minute, but you stay connected to your portfolio and avoid noise, syncing errors, and security trade-offs.

Let’s say you have:

• US Equity: 60% target
• International: 30% target
• Bonds: 10% target

Portfolio value: $100,000

After a strong market run:

• US Equity: $72,000
• International: $20,000
• Bonds: $8,000

Now:

• US Equity → +12% overweight (+$12,000)
• International → -10% underweight (-$10,000)
• Bonds → -2% underweight (-$2,000)

This is no longer a balanced portfolio.

It’s a much more aggressive one.

And most people don’t notice this shift.

After Rebalancing Using $10,000 of New Cash

You didn’t sell anything.

You just used new cash to move back toward your target.

There is more than one mathematically valid way to measure how far a portfolio has moved from its target. Alignfolio uses percentage-point drift - the difference between your actual allocation and your target allocation, in percentage points.

Another approach, sometimes called relative drift, measures the same gap as a proportion of the target itself (actual ÷ target, minus one). Relative drift is useful for comparing how far off-target different asset classes are when those targets are very different in size - a small satellite position and a large core holding can each be “off by 10%” in very different ways.

Neither approach is universally correct. They answer different questions:

• Percentage-point drift answers: “How much has my portfolio’s overall allocation shifted from what I intended?” It directly measures the difference between your intended allocation and your current allocation.
• Relative drift emphasizes how far each individual asset class has moved relative to its own target size - a useful lens too, particularly for smaller, tactical positions.

Alignfolio uses percentage-point drift to measure how far the portfolio has moved from its intended allocation. When recommending where to invest new cash, it calculates the dollar over- or underweight of each asset class because new contributions are made in dollars. Together, these two measures answer two different practical questions: “How far has my allocation drifted?” and “Where should my next dollar go?”

There’s no perfect rule, but these work well:

Alignfolio intentionally separates three different portfolio management decisions. Each one answers a different question and uses different information.

Directing new cash to underweight assets before selling anything:

• keeps the portfolio moving toward its target,
• minimizes unnecessary selling,
• can reduce realized capital gains in taxable accounts, since nothing has to be sold to make room for the contribution,
• and produces recommendations that are straightforward to understand and explain.

This is where most investors get stuck.

You don’t always need to sell to rebalance a portfolio.

Often, the simplest approach is using new cash correctly.

Want a full step-by-step walkthrough? See how to rebalance a portfolio using new cash.

Experienced investors usually:

• avoid unnecessary selling
• rebalance with new contributions
• keep the process simple
• potentially reduce taxes by avoiding capital gains from selling winners

It’s easier and often more tax-efficient.

Most tools overcomplicate rebalancing.

They tell you what to sell.

You don’t need perfect alignment.

You need:

• awareness
• a simple system
• consistency

If you’re:

• using Excel
• guessing allocations
• not tracking drift

There’s a simpler way.

Portfolio drift is quiet.

It slowly changes your risk without you noticing.

Most investors ignore it.

The ones who track it have a real advantage.

These sources support the broader portfolio management principles discussed on this page. Neither describes or endorses Alignfolio’s specific contribution-allocation methodology.

• Daryanani, G. (2008). Opportunistic Rebalancing: A New Paradigm for Wealth Managers. Journal of Financial Planning. Addresses when and how often to rebalance using tolerance bands - a question about monitoring and timing, not about how to divide a specific new contribution across several underweight asset classes.
• Vanguard. Rebalancing your portfolio: How to rebalance. Describes the general practice of directing dividends, interest, and new contributions toward underweight asset classes before selling.

These references describe the broader principles of portfolio management and rebalancing. Alignfolio’s contribution methodology is documented on this page.