For writing a story set in the Napoleonic Wars era, how should one use the currencies accurately? I read in novels they use currencies like shilling, pound, guinea, sovereign, four (or six-, etc) pence, etc. What is the conversion rate between these currencies, and when would one use such currency? Also, what would be the equivalent value today?
Seems like the questioner was asking for a bit more than just an idea of conversion rates, so here is some background on how the pre-decimal currency worked.
4 farthings = one penny
2 halfpennies = one penny
tuppence = colloquial two pence
thruppence = colloquial three pence
240 pence = one pound
6 pence = sixpence (aka a Tanner), or half a shilling.
12 pence = one shilling
two shillings = one florin
5 shillings = one crown
two shillings and sixpence = one half-crown
One guinea was latterly (and still is in horse racing) one pound and one shilling, although earlier in it's history its value varied according to the value of gold. For about the last 200 years it has only been a unit of account rather than an actual coin.
Likewise the Sovereign was a gold coin nominally valued at a pound, but its true value was whatever the value of the gold in the coin was worth that day. Bullion coins such as the Sovereigns were an investment to be stored for their own inherent value, rather than as a coin to be used when going out to buy goods.
Other common units of currency were:
The mark (merk in Scotland) - a very common unit of account, and not actually ever a coin (in England). A mark was 13 shillings and 4 pence.
The way to write prices in pre-decimal currency was generally as follows:
£10 5/- Ten pounds five shillings
£10 5/6 - Ten pounds five shillings and sixpence
£10 5s 6d was equally valid.
The National Archives have a converter.
But to be honest a couple of shillings was about enough to buy a decent meal, five shillings a night at an inn, a pound was quite a lot of money to be carrying around for a commoner and a penny could always buy you a bun (bread roll). That's all anyone really needs to know for narrative purposes. Getting into guineas, sovereigns and all that business will be too much detail for people just interested in the fiction.
Money's meaning changes over time; it is not a static category. For simplicities sake, lets assume that you learn how to use Lsd (£/s/d) figures, and work out a few of the more curious word name (tuppence).
Even then, you won't know how much money was worth in that era unless you read a lot of social history.
There are time values for money converters (for example, measuring worth), but their utility is limited unless you have a concrete understanding of the social meaning and value of, for example, not being able to eat in 1790 in Manchester. For which I recommend social history.
It would help greatly if you specified the social class you were writing about. Money's meaning for workers in the period was vastly different to money's meaning for the industrial, mercantile or landed (ie: aristocratic) bourgeoisie.