How is the investment gain/loss computed?

I purchased 2 Amazon shares 1 each at different times
1 share - May 2019 at £1500
1 share - Apr 2020 at £1940

Today the share was trading at £1900 and my investment gain/loss in +6% which is correct.
I would imagine that when I sell one share of Amazon at £1900 my investment gain should still read as +6% ? But what I see is around -2% which is computed based on the share I purchased on Apr 2020 and not based on the average purchase price.

So I would like to understand if that is the intended calculation?

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Normally the profit is calculated based on the average of those two purchases, which should be £1720.

It may or may not explain the difference; but did you look at the GBP/USD exchange rate difference? It could have played a part, plus 0.45% FX fee in both transactions

I can’t find the other thread about the P&L calculation but ultimately it’s a first in first out calculation rather than a cost average one which would remain constant when you sell.

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As you said, exchange rate and the FX rate doesn’t account for the 8% difference here, so i guess thats not the case here.

A cost average is the only way your purchase price should be calculated. Not including the price of the oldest shares when you sell, just doesn’t make sense.
FT should change this

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Adding to this so I don’t start another topic…

I bought £100 of Tesla at $799.67 and at 16:10 today Tesla is at $811.11

That is a 1.43% rise, but I’m down -0.15%…

What am I missing in my calculation? Even with the .45p fx fee, I should be at about +1.3%

Im sure it’s something simple…

You’ll have the bid / offer spread at least as a difference too. Not sure what the spread is on tesla but it’s not unreasonable to see it around 0.3%

Just looked on HL. At the moment it looks like this.

Maybe more price movement at the time?

I get the bid offer spread means I’m not guaranteed the price at point of sale but somewhere in between. But I actually got the share at $799 a few days ago… very odd and can’t quite figure it out.

Don’t forget about FX. GBP:USD in this case. Could be that FT is calculating your £ then vs £ now (rather than in $ terms). I’m not sure about this but it could explain it.