Their fingers and/or a calculator on their phone. Most real estate math is about 5th grade level addition, subtraction, multiplication and division, much of which we should be able to do in our heads in any event.

The only math required in residential real estate can be done in your head (or on your phone)  Anything more complicated means it is outside your area of expertise as a Realtor!

My Jethro Bodine 5th Grade Ciperhin' Notebook -- I like dem pictures.  

It use to be an abacus but now I use a calculator

The first tool I engage is my brain.  If I happen to need assistance then a $5 calculator works for me. I always check my work too. 

HP 12C  Financial Calculator

This formula:

sales price x my commission % = my commision

Calculator app on the phone or Google Now.  "Google, what is 20% of 640,000"  (for a conventional loan downpayment), "Google, what is 3% of 640,000" (for a 3% EMD).

Here are some fantastic tools:

Brain.

Fingers.

Calculator.

Abacus.

Google.

White board.

I use Karls Mortgage Calculator app on my phone and my title company apps for closing statement estimates.

Brain and calculator. For the investor side of things they should use a spreadsheet.

My iPhone or HP10B!

My head and then my Iphone

A calculator...

My brain, my fingers, and a calculator

I use the calculator on my smart phone these days.

I have a mortage loan app on my phone and a calculator. I use both of them. 

A sharp pencil and paper.

I still use a calculator but it is the one on my iphone. I no longer calculate payments for buyers. For my sellers I can do the simple math that is required with a pen and paper.

Calculator, spreadsheet, and there are about a bazillion RE online calculators out there now

a calcultor obviously but invest in a decent mortgage calculator. Its a good one when you are helping buyers figure out how much they can buy when they say "I can afford xx per month.  Or they want to know what their payment will be when they put xx down.  It's a good way for me to tell them "based on the last set of interest rates I received from my prefered lender it will be.... Then I send them to my lender.

Their brains should work best. I personally don't like memorizing formulas and those are mostly for commercial RE.

I will seriously naswer your question instead of being snarky. Patzaby.com has a ton of auto-calculating spreadsheets. Been using his products for 20 years, check them out. Great company and a great guy.

I dont math as a rule. I keep it simple. If math is required, WE do it together

Minnette Olsen I still use my trusted HP12C that I purchased in 1985. I also use an app on my iPhone called powerOne financial calculator.

Boy, not sure if I use a calculator to often but to figure my commission.

iphone has great stuff.

mortgage, closing costs for example.

It is good to have a financial calculator.

Make sure the display on the instrument is sufficiently large for the numbers to be used.

The calculator function on the cell seems to work nicely.

You can observe in the "REPORT" section of this blog I have a strong reliance on Excel.