Discount Rate vs. IRR
The IRR is the Discount Rate r* that makes Net Present Value NPV(r*)==0. What this boils down to is two ways of making the same kind of profitability calculation.  You can choose a project with NPV(10%)>0, or you can choose based on IRR>10%, and the idea is you get to the same set of projects.  That's if everything is well behaved mathematically. But that's not the end of this story of finance, math, and alphabet soup. For investments that have multiple positive and negative cash flows, finding that r* becomes solving for the roots of a polynomial in r*, so that there can be multiple roots.  Usually people use the lowest positive root but really it only makes sense for projects where NPV(r)>0 for r<r* and NPV(r)<0 for r>r*.   To try to help with your understanding, you can evaluate a real estate project with r=10%, find the sum future discounted cash flows, which is the NPV, and do the project if NPV>0.   Or, you can take the future cash flows of a project, find the NPV as a function of the rate r, and find r* where NPV(r*)==0. That r* is the IRR.  If IRR=r*>10% and the NPV function is well behaved as above, you can also do the project. When we don't have to worry about multiple roots, the preceding two paragraphs will select the same identical sets of projects as meeting the 10% return requirement.
I have answered your question in detail here https://stackoverflow.com/questions/12396422/apr-calculation-formula The annuity formula in FDIC document is at first finding PVIFAD present value annuity due factor and multiplying it with annuity payment and then dividing it by an interest factor of (1+i) to reduce the annuity to an ordinary annuity with end of period payments They could have simply used PVIFA and multiplying it with annuity payment to find the present value of an ordinary annuity In any case, you should not follow the directions in FDIC document to find interest rate at which the present value of annuity equals the loan amount. The method they are employing is commonly used by Finance Professors to teach their students how to find internal rate of return. The method is prone to lengthy trial and error attempts without having any way of knowing what rate to use as an initial guess to kick off the interest rate calculations So this is what I would suggest if you are not short on time and would like to get yourself familiar with numerical methods or iterative techniques to find internal rate of return There are way too many methods at disposal when it comes to finding interest rates some of which include All of the above methods use a seed value as a guess rate to start the iterative calculations and if results from successive calculations tend to converge within a certain absolute Error bound, we assume that one of the rates have been found as there may be as many rates as the order of the polynomial in this case 36 There are however some other methods that help find all rates by making use of Eigenvalues, but for this you would need a lengthy discourse of Linear Algebra One of the methods that I have come across which was published in the US in 1969 (the year I was born :) ) is called the Jenkins Traub method named after the two individuals who worked jointly on finding a solution to all roots of a polynomial discarding any previous work on the same subject I been trying to go over the Jenkins Traub algorithm but am having difficulty understanding the complex nature of the calculations required to find all roots of the polynomial In summary you would be better of reading up on this site about the Newton Raphson method to find IRR Thanks for this, great in depth answer. I had previously calculated a WACC and have used it for my discount rate.   As part of your last point on revenue vs. cash, I've set a accounts receivable period of 30 days, and then applied a factor of 30/365 * revenue to understand what portion of my revenue is not cash in hand. Does that make sense? &gt; Does it make sense to calculate the IRR based on the outstanding value of the project, or just use the cash flows paid out?  What is the outstanding value of the project based on? I'm guessing it is the PV of net cash flow?  The timing of each cash outflow (i.e. investment) is crucial to calculating a proper IRR because of time value of money. Putting in $x each year for 49 years will give you a different figure from putting in $49x in the first year and zero for the next 48 years because a larger figure is tied up for a longer time period. "Just to confirm, you don't pay interest when holding a bond, the issuer of the bond pays you interest. The idea of calculating ""present value"" is as you suggest. You discount future payments using an appropriate rate. These future payments include both the coupon payments you receive through the life of the bond and the principal repayment at the maturity of the bond - each should be discounted from the due date of the payment to today's date. A typical rate to use would be the interest you yourself could earn by investing elsewhere (this gives you some idea of how much it would cost to get those payments another way), or perhaps some standard rate, for example the interbank rates such as LIBOR or FEDFUNDS." For ESPP, the discount that you get is taxed as ordinary income. Capital gains is taxed at the appropriate rate, which is different based on how long you hold it. So, yes, if the stock is going up, As Sean pointed out they usually mean LIBOR or the FFR (or for other countries the equivalent risk free rate of interest). I will just like to add on to what everyone has said here and will like to explain how various interest rates you mentioned work out when the risk free rate moves: For brevity, let's denote the risk free rate by Rf, the savings account interest rate as Rs, a mortgage interest rate as Rmort, and a term deposit rate with the bank as Rterm. Savings account interest rate: When a central bank revises the overnight lending rate (or the prime rate, repo rate etc.), in some countries banks are not obliged to increase the savings account interest rate. Usually a downward revision will force them to lower it (because they net they will be paying out = Rf - Rs). On the other hand, if Rf goes up and if one of the banks increases the Rs then other banks may be forced to do so too under competitive pressure. In some countries the central bank has the authority to revise Rs without revising the overnight lending rate. Term deposits with the bank (or certificates of deposit): Usually movements in these rates are more in sync with Rf than Rs is. The chief difference is that savings account offer more liquidity than term deposits and hence banks can offer lower rates and still get deposits under them --consider the higher interest rate offered by the term deposit as a liquidity risk premium. Generally, interest rates paid by instruments of similar risk profile that offer similar liquidity will move in parallel (otherwise there can be arbitrage). Sometimes these rates can move to anticipate a future change in Rf. Mortgage loan rates or other interests that you pay to the bank: If the risk free rate goes up, banks will increase these rates to keep the net interest they earn over risk free (= Δr = Rmort - Rf) the same. If Rf drops and if banks are not obliged to decrease loan rates then they will only do so if one of the banks does it first. P.S:- Wherever I have said they will do so when one of the banks does it first, I am not referring to a recursion but merely to the competitive market theory. Under such a theory, the first one to cut down the profit margin usually has a strong business incentive to do so (e.g., gain market share, or eliminate competition by lowering profit margins etc.). Others are forced to follow the trend. Lots of good answers.  I'll try and improve by being more brief.  For each option you will pay different taxes: Index Fund:   Traditional IRA Roth IRA You can see that the Roth IRA is obviously better than investing in a taxable account.  It may not be as obvious that the traditional IRA is better as well.  The reason is that in the traditional account you can earn returns on the money that otherwise would have gone to the government today.  The government taxes that money at the end, but they don't take all of it. In fact, for a given investment amount X and returns R, the decision of Roth vs Traditional depends only on your tax rate now vs at retirement because X(1-tax)(1+R_1)(1+R_2)...(1+R_n) = X(1+R_1)(1+R_2)...(1+R_n)(1-tax) The left hand side is what you will have at retirement if you do a Roth and the right hand side is what you will have at retirement if you do traditional.  Only the tax rate differences between now and retirment matter here.  An index fund investment is like the left hand side but has some additional tax terms on your capital gains.  It's clearly worse than either. Lets imagine two scenarios: 1) You make 10.4k (40% of total income) yearly contributions to a savings account that earns 1% interest for 10 years. In this scenario, you put in 104k and earned 5.89k in interest, for a total of 109.9k. 2) You make the same 10.4k yearly contribution to an index fund that earns 7% on average for 10 years. In this scenario you put in the same 104k, but earned 49.7k in interest*, for a total of 153.7k. The main advantage is option 1) has more liquidity -- you can get the money out faster. Option 2) requires time to divest any stocks / bonds. So you need enough savings to get you through that divestment period. Imagine another two scenarios where you stop earning income: 1-b) You stop working and have only your 109.9k principal amount in a 1% savings account. If you withdraw 15.6k yearly for your current cost of living, you will run through your savings in 7 years. 2-b) You stop working and have only 20k (2 years of savings) in savings that earns 1% with 153.7k in stocks that earns 7%. If you withdraw your cost of living currently at 15.6k, you will run through your investments in 15 years and your savings in 2 years, for a total of 17 years. The two years of income in savings is extremely generous for how long it starts the divestment process.  In summary, invest your money. It wasn't specified what currency we are talking about, but you can easily find access to an investment company no matter where you are in the world. Keep a small amount for a rainy day.