The free online GST and GST Free Portion Calculator! Provided here for your use. Please make sure you report any issues with the calculator in the comments below. Enjoy your calculations!

I wrote this simple little script in JavaScript to help me calculate Australian GST (10%) today. Being such a useful little script, I thought I would post it.

Please feel free to use this script as your require, if you wish to have this script on your own website please post an acknowledgement and link back to this page.

**How to Calculate GST the hard way (using you know, numbers and shit)**

If you have something without GST, and you need it with GST, you add 10%. Which is easilest done on a calculator by multiplying your amount by 1.1 (which is really saying mister Calculator, give me 110% of this amount).

`$100 * 1.1 = $110`

To go the other way, let's say we have $110, which includes 10% GST already. Working out how much is the GST part is quickly done on a calculator by dividing by 11. Not 10 you say? No, 11. Because easiestly explained, before GST you had 10 parts of a number, the "extra" 10% is another "part". So we need to work out what that part is.

`$110 / 11 = $10`

Huzzah! We have the GST component of an amount! Now, we need to take that GST part from our original amount of $110 to get what the amount is without GST.

`$110 - $10 = $100`

Perfect :-)

But seriously, why do all this manual work when this GST calculator does it for you?

Last Updated 04/03/2014: Fixed floating point arithmetic (Loss of 1c on some calculations). Thanks to Michael for pointing it out.

Great app. I use it all the time. Straight forward. Saves me a lot of time.

the gst calculator is gr8 use it all the time thanx

Thanks again for all your work in this tool.

So incredibly useful. Don’t know how many times I’ve come back to it.

Thank you, simply explained! Now I can use these formulas in excel. Thank you.

do you have an app available for iOS (iphone) or should I bookmark this page

Not yet. It’s on my agenda of things to do but haven’t had time just yet.

I love this tool and great presentation. Book marked!

Thank you so much you have saved my scrawny neck.

Thank you so much for providing this – I find this very useful in my work

I have used your calculator before no worries but today it said “Unable to Calculate”. Total $405.20, GST stated $29.01, would not find the GST free portion.

Rounding issue and my calculation can’t work it out past a fraction of a cent.

Use gst calculated at $29.00 for the result.

You give the formula:

$100 * 1.1 = $110

The GST rate is 10%, so the “1.1” can be calculated as 1 + (rate/100).

But you also gave the formula:

$110 / 11 = $10.

How do you calculate the “11”?

Don’t worry. I worked it out.

GST = Total x (1 – (1 / (1 + (rate/100)))

So I guess 1 / 11 = (1 – (1 / (1 + (rate/100)))

Or

11 = 1 / (1 – (1 / (1 + (rate/100)))

Can you calculate GST on an individual item, then round down the figure and multiply it by the total quantity to get the GST value?

Qty 1,000 @ $2.79

GST on total invoice: 1,000 x $2.79 / 11 = $253.64

GST on individual item: $2.79 / 11 = .253636 you then round it down to $0.25 x 1,000 = $250.00

I know the difference is only $3.64 and I can manually change the GST, but are both these methods acceptable or should GST be calculated on the total of the invoice?

I hope this guy who rounds down, rounds up figures that are above the .5 mark. If he does, he is wasting his time and everyone elses, because the rounding up should offset the rounding down you would think.

Fantastic, you got me out of a jam today

this site is an AWESOME help i use it monthly….. thank you heaps!!!

Hi, I just need to know the calculation method for finding out the amount of GST that has been added when you don’t know what the initial amount was.

eg. the full amount with GST is 149.85, how do I work out what the GST was from this end amount?

I’m told the GST portion is13.62, and the beginning amount was therefore 136.23, but I need to know how it arrived to these amounts.

Thanks

$149.85 / 11 = $13.62 gst

$149.85 – $13.62 = $136.23 before tax.

The GST calculator does this for you above. Just enter $149.85 – click calculate GST.

The second ‘How to Calculate GST the hard way’ explains how GST is calculated (both forwards and back).

Thankyou, I use this site ALL the time, makes my life simple!

Hi Darren, i am all new to this GST, for seven shits the company i contract to takes $30 per shift for car usage =$210 then $12 per shift for what they call excess reduction = $84.00. then they take 5$ GST on the two added together…….

so my question is how do i calculate 5% on the total $294.00.

Thank You

SJ

Sorry, I don’t follow what you’re doing. But 5% of $294.00 is $14.70.

Why do you need 5%?