Know Your Financial Math
Please keep in mind that these are simple estimations and are not to be treated as precise technical calculations. They can be influenced by a number of factors and don’t take any personal information into account. The formulas help call attention to parts of your budget, but do not calculate exactly what you should expect.
The easiest and best place to start. Your cash flow is the total surplus or deficit you have each month after paying your expenses. If you find you are running a deficit most months, you need to cut your expenses down or find a way to boost your income.
Another easy formula, calculating how much a monthly (or weekly) expense will cost you over a whole year is an important insight for a budget. Paying $8 a month for a subscription may seem cheap, but you should realize it’s costing you $96 over the course of a year.
The EPA estimates that the average car owner uses about 500 gallons of gas a year (almost 700 if you drive a truck or SUV). While volatile gas prices make it impossible to project your exact gas expenses for a year, this formula makes it easy to understand how much a change in gas prices is worth: for every $0.01 gas drops, you could expect to save $5 annually.
Have you ever wanted a quick estimate of how long it takes for money to double? Try the “Rule of 72.” Just divide 72 by the annual growth rate of your account and you get an approximation of how many years it takes to double. (Example: 6 percent growth would be 72/6 = 12 years to double). If using this formula for investment account, remember that the market is unpredictable and average market performance does not guarantee future returns. Investments can be subject to losses, which will greatly change their nominal rate of return.
Although there are some major outliers, most new cars depreciate around 10% when driven off the lot and another 10% each year they are driven (for the first 5 years). So when looking at new cars, remember that most lose their value fast. Without a down payment, you’ll likely be underwater on the loan for the first year or two.
This equation is a bit more complex, but it’s pretty handy for people wondering how their rent cost compares to a 30‐year mortgage. Take 75 percent of the expected mortgage interest rate and add 3 percent to get the annualized rate of repayment. If you multiply this number by the initial mortgage amount, you get the annual cost. (Example: A 30‐year mortgage issued at 4 percent would have an annual repayment rate of (3+4×.75) = 6%. If the mortgage was for $200,000, you’d pay ($200,000×6%) = $12,000 a year ($1,000 a month) to stay on the 30 year schedule.) Keep in mind that this is an estimation of the mortgage costs only and does not include home insurance, mortgage insurance, property expenses or any of the other various costs of owning a home.