When travelling abroad, you often try to use some quick calculations for currency conversions, such that you can quickly in your head find what a certain price is in your own currency. From € to my local currency, kr, I e.g. do “multiply by 7 and add a bit” or from € to £ “take somewhat more than 10% off”. These calculations in your head often get imprecise, so it might be useful to have something better, maybe in your purse. For my local currency to €, having a printed copy of this:

does exactly that. Lookup € on the outside and convert to kr. on the inside. If something costs €29, I see there is a marker for 2.8 on the outside, then a somewhat larger marker and the number 3, so €2.9 is halfway between these two. On the inside, this is little before the 22 marker so €2.9 is little under 22 kr, hence €29 is little under 220 kr. Or to e.g. convert 550 kr to €, I find there is a marker for 55 on the inside (between the numbers 50 and 60), which on the outside is around 7.4. So 550 kr are equivalent to € 74.

Do you want to create one yourself? Just download the exchange_rate_circle and follow the instructions on slide 3.