How to calculate the ROI of a LEGO set — and how Cault does it for you

A LEGO set is fun. But in some cases, it's also a serious investment. A study of 2,322 sets sold between 1987 and 2015 calculated an average annualised return exceeding 11% — comparable to stock market performance over the same period. So how do you know if your collection is really gaining in value?

The basic formula

ROI (%) = ( Resale price − Purchase price − Fees ) / Purchase price × 100

Annualised ROI: the real comparative measure

A gross ROI of 163% over 8 years is very different from 163% over 2 years. To compare sets against each other — or against other investments — you need to calculate the annualised ROI.

ROI/year = Gross ROI / Number of years held

Example: 163% over 8 years → 163 / 8 ≈ 20.4%/year

How Cault determines your reference price

Before calculating ROI, you need a price. Cault reads 4 distinct metrics from BrickLink, updated daily:

The liquidity signal — how easy is it to sell?

A set can have a great ROI on paper but be very difficult to sell. Cault calculates a liquidity signal based on the sales/listings ratio over 6 months:

Ratio = Units sold (6 months) / Units currently listed

Sealed new: qty_sold_new  / qty_listed_new
Used:        qty_sold_used / qty_listed_used

The sealed premium — condition changes everything

An unopened set is structurally worth more than an opened one. Cault automatically calculates the sealed premium and contextualises it:

Sealed premium (%) = ( Sealed new price − Used price ) / Used price × 100

A high sealed premium (>40%) is a strong signal: buyers are willing to pay much more for an unopened copy, indicating sustained collector demand. A negative premium (<0%) is rare but can indicate a quality or image problem with the set.

Theme benchmark — your set vs the rest of the theme

Cault doesn't just give you your set's ROI in isolation. It compares it against the average of all sets in the same theme in our database.

For each set in the theme:
  roi_set = (market_price − retail) / retail × 100
  years   = today − retirement_date (or release year)
  roi_yr  = roi_set / years

Theme average = average(roi_yr) across all sets in the theme
                with valid market data (≥ 6 months old)

The 3 retirement phases

After its end of production (EOL — End Of Life), a LEGO set goes through 3 distinct phases. Cault detects them automatically and adapts its indicators accordingly.

Estimated break-even — when the set becomes profitable again

For sets in the mature phase whose used price is still below retail, Cault estimates a return-to-positive year. The calculation is based on the observed trajectory since retirement:

Opening baseline   = Retail price × 0.70
  (−30% = typical loss on opening a set)

Estimated annual gain = (Current price − Baseline) / Years since retirement

Years to break-even = (Retail − Current price) / Annual gain

Return-to-positive year = Current year + Years to break-even

Your overall portfolio ROI

Across your entire collection, Cault automatically aggregates:

Total invested value = Σ purchase_price of all your items
Total market value   = Σ market_price of all your items

Overall ROI (%) = (Market value − Invested value) / Invested value × 100

This calculation is available per collection (LEGO sets, LEGO minifigs) and globally across all collections. Here is an example portfolio of 3 sets:

Benchmarks to assess your performance

What Cault doesn't yet calculate automatically

Cault calculates the gross potential gain based on semi-real-time market prices. Two things remain manual for now:

• Selling fees — platform commissions (8–15%) and shipping costs are not yet automatically deducted. You can enter them in the transaction interface for a fee-net ROI.
• Taxation — above certain annual disposal thresholds, capital gains may be taxable. Cault does not perform tax calculations.

Integration of automatic fee estimates based on the chosen resale platform is on our roadmap.

Conclusion

Calculating the ROI of a LEGO set is accessible to everyone. The formula is simple: (resale − purchase − fees) / purchase. What changes with Cault is that you no longer have to do it yourself — and you have far more refined indicators than a simple percentage: liquidity signal, sealed premium, theme benchmark, retirement phase, estimated break-even.

Whether you're starting to collect with an investment mindset, or simply want to know if your collection is worth more today than yesterday — that's precisely what Cault was designed for.