Tracking a portfolio in a spreadsheet, and where it breaks down
A spreadsheet is the right tool for longer than people admit. Here is what it handles well, the four places it fails silently, and how to know when you have hit them.
August 15, 2026 · 4 min read
Luum exists because a spreadsheet stopped being enough. That is a genuine origin story rather than a marketing device, and it means this article is written by someone who maintained one for years and still thinks it is underrated.
A spreadsheet is the right tool for longer than most software companies would like you to believe. It is free, it handles any account type and any asset, and it encodes exactly the rules you decide matter. Plenty of people should not replace theirs.
What follows is where it actually breaks — not where it becomes tedious, but where it starts producing numbers that are wrong without telling you.
What a spreadsheet does well
Arbitrary assets. Private company shares, a rental property, a defined-benefit pension estimate, precious metals in a safe. No aggregation tool will cover all of these, and a spreadsheet does not care.
Rules you choose. If you want to value something a particular way, or exclude a category from your allocation, you write it. No product decides for you.
Total transparency. Every number has a visible formula. Nothing is computed somewhere you cannot inspect, which is more than can be said for most tools, including ours.
Durability. A CSV export will open in fifty years. Products get discontinued.
Where it breaks, in order of severity
1. Adjusted cost base across accounts
This is the one that costs money.
Canada pools identical securities across all your non-registered accounts into a single average cost. Maintaining that pool by hand requires you to correctly record every buy, every partial sale, every reinvested distribution, and every return-of-capital adjustment, across every institution, in the right order.
The failure mode is not that the formula breaks. It is that a T3 arrives in March with a box 42 amount, you are busy, and you do not go back and reduce the cost base. Nothing looks wrong. The error compounds silently for years and surfaces the year you sell.
2. Silent staleness
A spreadsheet does not know it is out of date. If you skipped entering a dividend in August, every figure derived from it — return, income, yield, allocation — is quietly wrong, and the sheet looks exactly as confident as it did when it was right.
Products that sync have a different failure mode: they break loudly, or they show you a stale-data warning. A spreadsheet's errors are always silent, which makes them harder to catch than more frequent but visible ones.
3. Returns that account for contribution timing
Computing a simple return is easy. Computing a time-weighted return properly means splitting your history at every cash flow, computing each sub-period, and chaining them.
It is doable in a spreadsheet. It is also the single most common place where people's formulas are subtly wrong — usually by treating a contribution as a gain, which inflates the result. If your spreadsheet has ever told you that you beat the index in a year you contributed heavily, this is the first thing to check.
4. Look-through and overlap
Knowing you hold 40% in two ETFs is easy. Knowing what those two ETFs both hold, and therefore what your actual concentration in any single company is, requires holdings data for each fund, refreshed as it changes.
There is no reasonable manual approach to this. It is simply outside what a spreadsheet can do without a data feed.
How to know you have hit the wall
Three reasonably objective signals:
- You have the same security in more than one non-registered account. Your ACB is now a cross-account calculation and the risk of getting it wrong has stepped up sharply.
- You have received a T3 with an amount in box 42 and are not certain you adjusted for it. Return of capital is the most commonly missed adjustment in Canadian investing.
- Reconciling takes more than an hour a quarter, or you have started skipping it. Skipped reconciliation is when the numbers begin to drift.
If none of those apply, your spreadsheet is fine and you can stop reading.
If you keep the spreadsheet
Record trades at the transaction level, not as monthly balances. Balances cannot reconstruct a cost base.
Keep a separate tab for distributions with the T3 box numbers, and reconcile it in March when slips arrive rather than in December.
Record the account against every transaction. A pooled ACB calculation needs to know which transactions were non-registered.
Export a CSV backup each year. A spreadsheet you cannot open is worth nothing, and whichever tool you eventually move to will want that history.
This article is educational and general in nature. It is not investment, tax, or legal advice, and it does not take your own circumstances into account. Verify tax treatment with the CRA or a qualified tax professional.