What is the PRU?
The average cost basis (PRU, from the French "Prix de Revient Unitaire") is the average acquisition cost of one unit of an asset held in a portfolio, factoring in every purchase made on that position.
How it is calculated
On every new purchase, the average cost basis is recalculated as a quantity-weighted average: the total amount invested (previous purchases plus the new one, fees included depending on the convention used) is divided by the total quantity held after the purchase.
Effect of a partial sale
Key takeaway
This is the convention PortefeuilleZen uses to stay consistent and predictable, rather than introducing a lot-valuation method (LIFO/FIFO) that would add complexity without a clear benefit for personal wealth tracking.
What it's used for
The average cost basis is used as a reference to estimate an unrealized gain or loss (current value versus average cost), but should not be confused with the portfolio's actual performance, which accounts for cash flows, fees, dividends and holding period.
Attention
Worked example
You buy 10 units at €100 with €5 of fees (cost: €1,005), then 10 units at €120 with €5 of fees (cost: €1,205). You hold 20 units for €2,210: the average cost basis is €110.50. If you then sell 5 units, the remaining 15 keep an average cost basis of €110.50.
In PortefeuilleZen, acquisition fees and taxes are included in the cost. An opening position starts its history on the declared date with the average cost you provide: no past purchase is ever invented.