Gas bottles don't come with gauges, so how do you know if they are getting close to empty? You don't want to refill to early, as you won't get value in the refill (refilling is a flat rate charge), and you don't want to leave it too late and be stranded without gas?

The most accurate way to measure remaining capacity is to weigh the bottle. 

1. Start by removing the bottle from its holder/enclosure
2. Find the "Tare Weight" of the bottle – this is marked on the top of the bottle as "T.C." or "T.W.". Examples are 8.1 kg (common for a "9 kg" cylinder), 5.2kg (common for a "4.5 kg cylinder)
3. Find the maximum gas capacity of the bottle. This is not marked simply on the bottle, but usually you know your bottle is a "9 kg" or 4.5 kg". This is the weight of the gas it can carry at full load.
   1. If you don't know this capacity, you can calculate it.
   2. Find the "W.C." marking on the bottle. (Located next to the "T.C." or "T.W."). This is the "Water Capacity". E.g. for a "9 kg" bottle, the "W.C." is 22.0kg.
      1. This is the volume of water the bottle can hold.
      2. By law, the gas bottle can only be filled to a certain pressure.
      3. The normal propane or propane/butane mix at the specified pressure will weigh approximatey 41% of the water capacity..
      4. E.g. 41% of 22kg = 9kg
4. Weigh the bottle – using a set of bathroom scales is normally the easiest.
   1. E.g. Your current "9 kg" bottle weighs 11kg. 
   2. Take off the tare weight. E.g. 11kg – 8.1kg = 3.9kg.   You have 3.9kg of gas remaining from your 9kg capacity = 43%