Expense Ratio Math Examples
Last verified: 2026-06-21
Expense ratio math is one of the cleanest beginner investing lessons because the numbers are visible. An expense ratio is the annual fund cost expressed as a percentage of assets. It does not predict returns, but it does show the drag a fund charges before the investor keeps the rest.
This Bucko Library page is educational. It is built for research, journaling, scenario analysis, guardrails, and review. It is not personal portfolio guidance or a recommendation to buy, sell, trade, or avoid any security.
Source note: evergreen cost-math framework. Fund fees, transaction costs, tax treatment, and product details should be verified in the current fund prospectus or official materials.
The plain-English version
If a fund has a 0.10% expense ratio, the rough annual cost is ten cents per $100 invested. If a fund has a 1.00% expense ratio, the rough annual cost is one dollar per $100 invested.
That sounds small until the account size and time horizon grow.
Annual cost ≈ investment amount × expense ratio
The formula is simple. The discipline is comparing costs before getting distracted by branding, recent performance, or a complicated fund description.
Example 1: $10,000 invested
Compare two funds:
- ▸Fund A expense ratio: 0.05%.
- ▸Fund B expense ratio: 0.75%.
- ▸Investment amount: $10,000.
Approximate annual cost:
Fund A: $10,000 × 0.0005 = $5
Fund B: $10,000 × 0.0075 = $75
Difference: $70 per year before compounding effects
The question is not “is Fund B bad?” The better question is: “What extra value would need to exist to justify the extra cost?”
Example 2: $100,000 invested
Now use the same expense ratios with $100,000:
Fund A: $100,000 × 0.0005 = $50
Fund B: $100,000 × 0.0075 = $750
Difference: $700 per year before compounding effects
As the account grows, small percentages become real dollars. That is why expense ratio review belongs in a portfolio checklist.
Example 3: cost plus behavior
Fees are not the only issue. A cheap fund can still be a poor fit if the investor cannot stick with the plan. An expensive fund can still be inappropriate if the pitch is complicated and the review process is weak.
A better review asks:
- ▸What does the fund own?
- ▸What is the expense ratio?
- ▸How does it compare with similar alternatives?
- ▸Is there turnover, tax friction, or concentration risk?
- ▸What would make this fund get replaced later?
Expense ratio is one input. It is not the entire decision.
Common mistakes
- ▸Looking at recent returns before checking the cost.
- ▸Comparing funds in different categories as if they are the same.
- ▸Ignoring account size when thinking about small percentages.
- ▸Forgetting taxes, spreads, commissions, or platform fees.
- ▸Assuming a higher fee automatically means better research or better outcomes.
Costs are one of the few parts of investing that are knowable up front. That makes them worth documenting.
Bucko-style checklist
Before adding a fund to a watchlist, write down:
- ▸Fund name and ticker.
- ▸Expense ratio from the current official material.
- ▸Approximate dollar cost at the planned allocation size.
- ▸Category comparison.
- ▸Turnover, tax-friction, or concentration notes.
- ▸Reason the fund belongs in the portfolio.
- ▸Review trigger for replacing or reducing it.
Bucko fits here as an educational research and review workspace. Save fee notes, compare similar funds, tag cost friction, and review whether the fund still matches the original role.