<< payments <- function(loan, apr, months) { rate <- 1 + apr / 100 / 12 loan * rate^months * (rate - 1) / (rate^months - 1) } amortize <- function(loan, apr, months, payment = payments(loan, apr, months)) { rate <- 1 + apr / 100 / 12 month <- 0:months balance <- loan * rate^month - payment * (rate^month - 1) / (rate - 1) complete <- match(TRUE, balance <= 0.01) balance <- ifelse(month < month[complete], balance, 0) principal <- loan - balance interest <- payment * month - principal interest <- ifelse(month < month[complete], interest, interest[complete-1]) amrt <- list(month = month[-1], balance=balance[-1], principal = principal[-1], interest = interest[-1], paid = principal[-1] + interest[-1], loan=loan, payment=payment, apr=apr) class(amrt) <- "amortization" return(amrt) } plot.amortization <- function(amrt, ...) { icol <- rgb(189, 215, 231, maxColorValue=255) pcol <- rgb(8, 81, 156, maxColorValue=255) amrt\$paid <- amrt\$paid / 1000 amrt\$principal <- amrt\$principal / 1000 amrt\$interest <- amrt\$interest / 1000 year <- amrt\$month / 12 par(mar = c(4, 1, 1, 5)) plot(0, 0, xlim=range(year), ylim=range(amrt\$paid), type="n", las=1, yaxt="n", main="", cex.main=3, xlab="Year", cex.axis=1.5, cex.lab=1.5, bty="n", ...) axis(4, las=1, cex.axis=1.5, at=pretty(amrt\$paid), labels=paste("\$",pretty(amrt\$paid),"k",sep="")) polygon(c(year,rev(year)), c(amrt\$paid, rev(amrt\$principal)), border=NA, col=icol) polygon(c(year,max(year),0), c(amrt\$principal,0,0), border=NA, col=pcol) text(x=0,y=max(amrt\$paid),adj=c(0,1), cex=2.5, col=icol, font=2, paste("interest \$", round(max(amrt\$interest)), "k", sep="")) text(x=0,y=max(amrt\$paid),adj=c(0,2.5), cex=2.5, col=pcol, font=2, paste("principal \$", round(max(amrt\$principal)), "k", sep="")) } # Default quantities dloan <- 150000 drate <- 4.5 dyears <- 30 dpaymt <- payments(dloan, drate, dyears*12) # Read POST data, draw graphic if(!is.null(POST)) { floan <- as.numeric(POST\$loan) if(length(floan) == 1 && !is.na(floan) && floan > 0) dloan <- floan frate <- as.numeric(POST\$rate) if(length(frate) == 1 && !is.na(frate) && frate >= 0) drate <- frate fyears <- as.numeric(POST\$years) if(length(fyears) == 1 && !is.na(fyears) && fyears > 0) dyears <- fyears dpaymt <- payments(dloan, drate, dyears*12) fpaymt <- as.numeric(POST\$paymt) if(length(fpaymt) > 0 && !is.na(fpaymt) && fpaymt >= dpaymt && fpaymt < dloan) dpaymt <- fpaymt } amrt <- amortize(dloan, drate, dyears*12, dpaymt) amrtpp <- amortize(dloan, drate, dyears*12, dpaymt+1) filename <- paste(paste(sample(letters, 16, replace=TRUE),collapse=''), 'amort.png', sep='', collapse='') dirname <- '/srv/www/biostatmatt.com/public_html/R/MortCalcCache/' png(file=paste(dirname, filename, sep='', collapse=''), type="cairo", height=400, width=600) plot(amrt) dev.off() setContentType("text/html; charset=utf-8") >> MortCalc
MortCalc

loan amount:
interest rate:
years:
min. payment:
payment:
Mortgage Amortization

#### What is this?

The dark blue area in this graphic represents the total amount of money paid towards the principal of your home loan. When the amount paid towards principal equals the amount you borrowed, then the loan is considered repaid. The light blue area represents the total amount of interest paid on the loan. Interest is the money you pay the lender for loaning you money. The total interest is \$<> on a <> year loan of \$<> at %<> interest, making monthly payments of \$<>. That's %<> of the loan amount!

The dark and light blue areas combined represent the total amount you pay over the life of the loan. If you make more then the minimum monthly payment, your loan will be repaid before <> years, and there will be a region where the light and dark blue areas are flat (because you don't make payments towards principal or interest after the loan is repaid). You will also pay less in interest. Paying just \$1 dollar extra per month would save you \$<> in interest over the life of the loan. Update the 'payment' box under 'MortCalc' to see how larger monthly payments will affect your loan.