An mBTC, or MilliBitcoin, is equivalent to .001BTC or one-thousandth of a bitcoin.
You can also manually compute for it, if you wish, by multiplying by 1000 (e.g. the amount you wish to purchase x price per mBTC x 1000).
Example: calculation BTC price reference by mBTC price
If you would like to buy amount 0.3 BTC and mBTC price is 125 THB/mBTC.
So, you will need to pay 0.3 x 125 x 1,000 = 37,500 THB for 0.3 BTC
Example: compare mBTC to BTC reference by BTC price
If you have 0.4 mBTC and would like to know its value compare to BTC price.
But the price of 1 BTC is 120,944 THB at the moment and 1 mBTC equal to 0.001 BTC
So, 0.4 mBTC is 0.4 x 0.001 = 0.0004 BTC and its value will be 0.0004 x 120,944 = 48.3776 or around 48 THB.