I thought these handy, as I'm sick of running off to look up a conversion table, 'cos my brain is too stupid to remember it. Note that these values are rounded and provide an approximate only. The range was that thought useful for aquarium use. (make this a 'Sticky' please Bettina).

Temperature
Far: 66...68...70...72...73...75...77...79...81...82...84...86
Cel: 19...20...21...22...23...24...25...26...27...28...29...30

Volume
US Gal: 10....20....30....40....50....55....60....70
UK Gal: 08....17....25....33....42....46....50....58
...Litre: 38....75...114..151..190..208..227..265

Length
Imp.: 12in..........18in..........2ft...........3ft...........4ft...........5ft
Met.: 305mm.....457mm.....610mm....914mm....1.22m.....1.52m

this may not work for people using imperial measurements (inches and feet) but for those that use or can use metric (cm and metres) i have learned a good way of working out the volume and even the weight of water in a fish tank.

basicaly if u measure a fish tank using cms by multiplying the 3 measurements and deviding by 1000 then u can tell how many litres it holds. plus as 1 litre of water equals 1 kilogram in weight u can get an idea of how heavy the mass of water is. which is handy to know if u want to make sure u have stong enough supports to hold the tank up

for example if i measure my long 40g tank, not including glass and measuring from the top of the water line to the bottom, not the tank itself i get the following ...

39cm x 39cm x 99cm

multiply it together ... 39 x 39 x 99 = 150579

devide the total by 1000 ... 150579 / 1000 = 150.6 (rounded up)

this mean my tank actualy holds 150.6 litres of water and not 180 as the manufactures say.

i would also take 10% off this figure to account for rocks wood and plants, so my tank realy holds ... 150 - 15 (10%) = 135 litres of water

if i want to i can devide the number of litres by 4.55 and this will give me the amount of UK gallons ... 135 / 4.55 = 29.6 gallons (uk)

for those using US gallons u need to devide by 3.79 instead of 4.55. hence my tank in US gallons would be 135 / 3.79 = 35.6 gallons (us)

so as can see if i was to treat my fish tank with medicine for example, instead of putting in enough teatment for 40g i should only put in enough for 30g. an important thing to know as if i put in enough med for 40g i would be overdosing my tank by 33% which could endanger my fish more than help them.