Automate Savings by Percentage on varying paychecks?
When I have been faced with this sort of situation I have done the split at the bank.  They had the ability to recognize the deposit as a payroll transfer and split it the way I wanted. I put a specific amount of money into checking, another amount of money into the mortgage, and a specific amount of money into another fund. The balance, whether it was $1 or any other amount, went in to savings. That meant that I transferred the amounts I needed to pay my budgeted living expenses and what ever I made above that went to savings. In months I made extra, more was available to be saved. You just need to average out the weekly hours and income over the year. So if his yearly income is $100,000 p.a. then this would average out to $2000 per week of which 15% would be $300 per week. It does not have to be exactly 15% per week as long as over the long run your saving your target 15%. If he gets a pay rise you can include this in the saving plan. Say he gets a 5% increase in pay you would increase the $300 per week by 5% to $315 per week.
"I've tried Mint, and I've tried Quicken. Now, I think Quicken is an annoying, crashy little piece of software, but it is also quite capable; overall I think it has the features you want. You can enter your bills, broken down by category, in advance. You can enter your paychecks, broken down by category (gross income, federal income tax, state income tax, social security, SDI, transfers to tax-protected 401(k) account, etc) in advance. You can enter in your stock trades and it can tell you how much you'll need to end up paying in capital gains taxes. You can even enter in your stock option vesting schedule in advance (it's a royal pain because you can't go back and change anything without deleting everything, but you can do it). It'll forecast your bank account balance in all of your bank accounts in advance with a shiny chart. It'll even model your loans, if you set it up right. I didn't do too much with the ""budgeting"" tools per se, but the account-balances-daily features sound like the closest thing to what you're looking for that's likely to exist. The only thing that's a trifle tricky is that transfers from one account to another may take multiple days (hello, ACH) and you'll have to decide whether to record them at departure or arrival." neobudget.com is a website that does exactly what you are describing. It is set up for electronically using the envelope system of budgeting.   Disclosure: neobudget was founded by a former coworker of mine. "The short answer is ""yes"", paying more towards the loan as soon as a you can will reduce the interest. There are calculators or you could work up a spreadsheet using the specialized family of functions: PMT, PPMT, IPMT. My personal view: The amount of interest you offset in this manner is going to be fairly small (I'm going to guess less than $5 or $10 a month, but I haven't done the math). I would say what is more important is to automate your payments at a comfortable level, while making sure your other obligations are taken care of. Then add an extra payment when you save up a chunk of money to pay towards it. Make sure you never miss a payment. That means making sure you set up emergency fund to cover the payments if you lose your job or need to visit a sick family member for a while or the car breaks down or ...." "I currently use Mint for this, which I see that you have already disqualified but not why you have disqualified it.  Set ""budgets"" for how much you want to spend on what type of expense, and then be sure to assign expenses to a budget as they come in. Mint actually learns what expenses go to each budget and eventually does it automatically." "There is a saying in business: what gets measured gets done. Track every expense you make. Later, look over what you have learned. If 5% of your total budget is going to something frivolous, maybe you could halve it? If 1% or 0.1% is going to that frivolous expense, there's not much to be gained even by eliminating it. If you spend $200/mo on coffees, dropping those will help. If you spend $10/mo on coffees, you need to look elsewhere for your big savings. Have a target: I want to put $X into savings each month. Therefore I can only spend $Y. What do you have to change about last month's spending patterns to get down to $Y? Where are the easy targets for you? They will be different than the easy targets for me. What absolutely cannot change for you? Once you know the costs of what you're doing, you will know where it's possible to save, and where it's ""worth it"" to economize." "While I know some people prefer handling things more ""manually"", I really like automating everything possible. To the greatest extent possible, my deposits automatically go into my Checking account and my bills automatically withdraw from it. That way, I never have to worry about accidentally paying a bill late, and my financial life just runs mostly on its own. There are a couple things to be wary of when automating one's financial life:" To me, this question is really about setting and meeting goals. The process is the same, whether it's about exercising regularly, or saving, or whatever. You need to have clear, personally-relevant reasons for doing something.  Write down: Exactly why you want to save. It may seem trivial, but if you can't visualize the prize it's hard to stay motivated. How much can you afford to save? Use something like Mint.com to find out your real monthly expenses, as opposed to what you think you're spending. Also, don't get overzealous... leave yourself some money for small luxuries and unexpected expenses so you don't feel like a miser. Saving should be a joy, not torture. Automate the saving process. Set up an automatic transfer to move the amount you figured out in step 2 to your savings account on the same date you get paid. This is very important. By saving early you ensure there will be enough money to save. If you wait until the end of the month, there will usually not be anything left. Don't you dare touch your savings! (Except in a real emergency) If you must dip into your savings, immediately create a plan to put it back as soon as possible. Also, get into the habit of reading personal finance books, blogs, sites, etc. I recommend authors like Robert Kiyosaki, and Suze Orman. Good luck! To avoid having it become overly complicated, I suggest it be run as would a mutual fund. Mutual funds transact each evening to set a price. Transactions for purchases or sales are done at that price each evening.  Initially, you have a dollar amount invested for each person. You can calculate the percent of the 'fund' each has, and, assuming the total is under $10K, 7 digits after the decimal accuracy is enough to track each share to the 1/10 cent.  When new money is added, that night, you calculate the exact value of investments, and add the new funds, so each person now has a smaller share of the larger fund.  If you wish, you can normalize this to 'share value' so my initial investment of $1000 is 100 shares regardless of the total amount invested. Then when new money comes in, the 'shares' increase as well. This may feel better as a declining percent may just seem awkward, even though that's the case. Not sure. I know Tim Ferriss wrote a lot about automating business processes in his book Four-Hour work week. Which I've applied the ideology myself and always looking to automate or delegate a routine task that doesn't necessarily have to be done myself. What about you? Are you trying to automate? "The principle to follow is called ""pay yourself first"". Have your savings deducted from your paycheck before it hits your checking account. You spending will change to accommodate the reduction. If you have a 401(k) available from your employer, start saving some money via that. If not, figure out a way to have something moved out of your checking account to a separate savings account when your paycheck hits. Then as you get raises, up the amount of automatic savings by half of the raise. You will find this hurts less than you think and it will let you build an emergency fund, which is the first thing you need. When the emergency fund is 6 months of normal spending, then you can start to invest." Do you have direct deposit of your paycheck? If so, almost every employer will allow you to split the paycheck into two accounts. You could open one account for savings, and one for spending. Put $x from each paycheck into the savings account, and the rest into the spending account. Keep the savings account totally separate, with its own ATM card. There should be no way to get money out of this account except by using the SEPARATE ATM card. Now, get a dish of water. Put the ATM card in the dish of water. Put the dish of water in the freezer.  If you are ever tempted to spend your savings, you'll have to wait for the block of ice to defrost. Hopefully, while the ice is defrosting, the urge to waste money will pass :) A couple ideas: Use excel - it has an IRR (internal rate of return) that can handle a table of inputs as you describe, along with dates deposited to give you a precise number.  Go simple - track total deposits over the year, assume half of that was present in January. So, for example, your account started the year with $10k, ended with $15k, but you deposited $4k over the year. It should be clear the return (gain) is $1k, right? But it's not 10%, as you added during the year. I'd divide $1k/$12k for an 8.3% return. Not knowing how your deposits were structured, the true number lies between the 10% and 6.7% as extremes. You'll find as you get older and have a higher balance, this fast method gaining accuracy, as your deposits are a tinier fraction of your account and likely spread out pretty smoothly over the year anyway. Interesting. When you say DIY you mean pencil and paper. For most of us the choice came down to using a professional vs using the software.  Your second bullet really hits the point. The tax return is a giant spreadsheet with multiple cells depending on each other. Short of building my own spreadsheet to perform the task, I found the software, at $30-$50, to be the happy medium between the full DIY and the Pro at $400+.  With a single W2, and no other items, the form is likely just a 1040-EZ, and there shouldn't be any recalculating so long as you have the data you need. Pencil/paper is fine.  There's no exact time to say go with the software, except, perhaps, when you realize there are enough fields to fill out where the recalculating might be cumbersome, or the need to see the exact tax bracket has value for you.  You are clearly in the category that can fill out the one form. At some point, you might have investment income (Schedule D) enough mortgage interest to itemize deductions (Schedule A) etc. You'll know when it's time to go the software route.  Keep in mind, there are free online choices from each of the tax software providers. Good for simple returns up to a certain level. Thanks to Phil for noting this in comments.  I'll offer an anecdote exemplifying the distinction between using the software as a tool vs having a high knowledge of taxes. I wrote an article The Phantom Tax Zone, in which I explained how the process of taxing Social Security benefits at a certain level created what I called a Phantom Tax Rate. I knew that $1000 more in income could cause $850 of the benefit to be taxed as well, but with a number of factors to consider, I wanted to create a chart to show the tax at each incremental $1000 of income added. Using the software, I simply added $1000, noted the tax due, and repeated. Doing this by hand would have taken a day, not 30 minutes.  For you, the anecdote may have no value, Social Security is too far off. For others, who in March are doing their return, the process may hold value. Many people are deciding whether to make their IRA deposit be pre-tax or the Post tax Roth IRA. The software can help them quickly see the effect of +/- $1000 in income and choose the mix that's ideal for them. Take a certain percentage of your income (say, 10%, but more is better if you can) and put it aside with every paycheck. Some employers will even allow you to direct deposit your paycheck into two different accounts and you can specify a certain amount or percentage for the second account. Your savings will go directly into a separate account as if you never had it in the first place. Consider your savings untouchable as spending money. Watch it grow. There's no other secret, you just have to do it! "For a personal finance forum, this is too complicated for sustained use and you should find a simpler solution. For a mathematical exercise, you are missing information required to do the split fairly.  You have to know who overlaps and when to know how to do the splits.   For an extreme example, take your dates given: Considering 100 days of calculation period,  If Roommate D was the only person present for the last 10 days, they should pay 100% of the grocery bill as they are the only one eating.   From your initial data set, you can't know who should be splitting the tab for any given day. To do this mathematically, you'd need:  But don't forget ""In Theory, Theory works.  In Practice, Practice works."" Good theory would say make a large, complicated spreadsheet as described above.   Good practice would be to split up the costs in a much, much simpler way."