If you're someone who has a budget in the first place, you're actually ahead of most Americans. In a recent U.S. Bank study, almost 60% of U.S. adults admitted they don't follow a budget, despite the fact that it's one of the easiest ways to manage one's finances. That said, having an inaccurate budget will only get you so far if your goal is to stay on top of your money. Here are five mistakes in particular you'll want to avoid at all costs.

1. Guessing at numbers rather than using hard data

The purpose of having a budget is to see where your money is going and where you might find room to cut costs. But if you're working with the wrong set of numbers, your budget won't help you achieve those goals. That's why when breaking out your spending categories, you really can't just guess at numbers, or even rely on your memory to see what you spent over the past year. Instead, pull up your old credit card and bank statements to see what you really spent, on average, for each category that's going into your budget.

Man working on a budget

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2. Forgetting one-time expenses

We're all used to having monthly expenses, whether it's rent payments or a cellphone bill. But it's easy to forget about one-time expenses, like your homeowner's insurance or warehouse club renewal. As you sit down to write up your budget, comb through those credit card and bank statement once again in search of expenses that aren't recurring, and factor them into your monthly spending. If, for example, you pay a yearly $1,200 auto insurance bill, you should allocate $100 a month toward that expense. Then, for each of the 11 months you aren't paying that bill, stick that $100 in a dedicated savings account so that it's available once that lump-sum payment comes due.

3. Assuming your costs will be fixed and not variable

Many of the costs you'll incur throughout the year, like your car payment or transit pass, will stay the same month after month. But while you might think you'll spend virtually the same amount of money each month on other expenses, like utilities and food, the opposite might end up being true. And if you don't account for those upticks in spending, you're likely to come up short.

For example, you'll probably spend more to heat your home in the winter, and to cool your home in the summer, than you will on heat or electricity during months with a more moderate climate. Similarly, if you tend to host a number of barbecues over the summer, or several holiday parties in December, you'll need to either give yourself a higher grocery allowance for those months or otherwise factor those increases in spending into your monthly totals throughout the year.

4. Spending your entire paycheck on living expenses

You've run the numbers, accounted for every last expense category that applies to you, compared that total to the amount you take home each month, and celebrated the fact that you've managed to break even. Not so fast. While not spending more than what your monthly paycheck delivers is a good start, maxing out your take-home pay is a big mistake for two reasons. First, it means you won't have any money left over to put into a savings account or retirement plan, but just as importantly, it means you'll be leaving yourself zero wiggle room for unplanned expenses.

At a minimum, you should always make sure your budget leaves you enough room to put aside 10% of each monthly paycheck. This way, even if you max out the rest of your income for that time, you'll still have some savings to show for. And if you have a month where you do exceed your allocation in another category, you can scale back your savings temporarily to compensate. It's not ideal, but it's a better bet than racking up debt.

5. Neglecting to update your budget when things change

Perhaps you've been paying a pretty low rate for your cable plan thanks to an introductory offer you capitalized on. But if your one-year intro rate is up and your bill is about to jump $30 a month, that's something you'll need to account for. While you don't necessarily need to rewrite your budget every month, it pays to examine it once a quarter to ensure that your numbers are still applicable. Along these lines, if you tack on a new expense category (say, by joining a gym or signing up for a video streaming service), be sure to adjust your budget right away. You may need to scale back on an existing spending category to make room for a new one.

The more thought you put into your budget, the better it'll help you manage your money. If you steer clear of these mistakes, you're less likely to overspend and let your savings goals fall by the wayside.

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