---
title: "Markup Calculation"
id: 57168
type: "computer_media"
slug: "markup-calculation"
url: "http://localhost/computer_media/markup-calculation/"
markdown_url: "http://localhost/computer_media/markup-calculation.md"
published_at: "2024-10-04T01:11:53+00:00"
modified_at: "2026-03-30T21:19:15+00:00"
author: "David Anderson"
featured_image:
  url: "http://localhost/wp-content/uploads/2024/10/117_MkUp.png"
excerpt: "A handy retail pricing tool that calculates selling prices using either cost-based or sales-based markup — two different formulas with a single percentage input."
category:
  - name: "Archived Media"
    slug: "archived-media"
    taxonomy: "category"
    url: "http://localhost/category/archived-media/"
post_tag:
  - name: "Downloadable"
    slug: "downloadable"
    taxonomy: "post_tag"
    url: "http://localhost/tag/downloadable/"
  - name: "TS 1000"
    slug: "ts1000"
    taxonomy: "post_tag"
    url: "http://localhost/tag/ts1000/"
model:
  - name: "Timex/Sinclair 1000"
    slug: "ts-1000"
    taxonomy: "model"
    url: "http://localhost/model/ts-1000/"
genre:
  - name: "Business"
    slug: "business"
    taxonomy: "genre"
    url: "http://localhost/type/business/"
media_contents:
  - id: 56733
    title: "Timex Sinclair Public Domain Library Tape 1002"
    type: "computer_media"
    url: "http://localhost/computer_media/timex-sinclair-public-domain-library-tape-1002/"
media_type: "Program"
mediadate: "198x"
images:
  - url: "http://localhost/wp-content/uploads/2024/10/117_MkUp.png"
media_type_tags: "Business"
---

# Markup Calculation

This program calculates retail selling prices based on either a cost-based or sales-based markup percentage. The user enters a cost, a desired markup percentage, and selects whether the markup should be applied on cost (additive: S = C + C×R) or on selling price (divisive: S = C ÷ (1−R)). Rounding to two decimal places is achieved with the classic `INT(S*100+.5)/100` idiom. The program loops to allow multiple calculations before stopping.

***

## Program Analysis

### Program Structure

The program is a straightforward single-file BASIC application with a linear flow broken by conditional branches. It can be divided into four logical phases:

1. **Input phase** (lines 4–41): Title display, cost entry (`C`), and markup percentage entry (`R`).
2. **Markup basis selection** (lines 120–170): Asks whether markup is on Cost (`"C"`) or Selling price (`"S"`), with a validation loop for invalid input.
3. **Calculation phase** (lines 180–244): Two branches compute the selling price `S` and set the label string `B$`.
4. **Output and repeat** (lines 245–290): Prints results, then offers to loop back to line 10 for another calculation.

### Markup Formulae

The program implements two standard retail pricing calculations:

| Basis | Code | Formula | Meaning |
| --- | --- | --- | --- |
| On Cost | `A$="C"` | S = C + C×R | Selling price is cost plus a percentage of cost |
| On Selling Price | `A$="S"` | S = C ÷ (1−R) | Markup is a percentage of the final selling price |

The distinction matters practically: a 50% markup on cost yields a 33% margin on selling price, while a 50% markup on selling price requires doubling the cost price.

### Key BASIC Idioms

- **Percentage normalisation** (line 50–60): If the user enters a value ≥ 1 (e.g. 25 meaning 25%), line 60 divides by 100 to convert to a decimal fraction. Values already less than 1 are assumed to be pre-converted, bypassing division.
- **Banker’s-style rounding** (lines 190, 230): `INT(S*100+.5)/100` rounds the result to two decimal places — the standard approach for currency on this platform where `ROUND` is not a built-in function.
- **String label variable** (`B$`): Lines 200 and 240 set `B$` to `"COST"` or `"SALES"` respectively, so a single `PRINT` statement at line 255 handles both cases without duplication.
- **Echo-printing inputs** (lines 21, 41, 131): After each `INPUT`, the entered value is immediately `PRINT`ed back. This is characteristic of the ZX81/TS1000 `INPUT` behaviour where the prompt line is cleared after entry, serving as a confirmation display.

### Notable Techniques

The validation loop at lines 120–170 is a simple but effective pattern: after printing the prompt and reading `A$`, two `IF` tests branch away on valid input (`"C"` or `"S"`), while any other value falls through to print an error message and `GOTO 120` to retry. This avoids the need for a structured loop construct.

Line 245 contains a blank `PRINT` for spacing, and line 258 likewise, giving the output a cleaner layout. `TAB(5)` is used in lines 4, 250, and 255 to indent the output for readability.

### Bugs and Anomalies

- **Case sensitivity**: The comparisons at lines 140 and 150 require uppercase `"C"` and `"S"`. On a physical keyboard this is typical, but a lowercase entry will always trigger the “INVALID CODE” message with no case-folding applied.
- **Zero or negative markup**: If `R=0` is entered, the calculation proceeds correctly (selling price equals cost), but if `R≥1` and the user actually meant a decimal (e.g. entering `1` to mean 100% markup), the division at line 60 converts it to `0.01` (1%), which may not be the user’s intent. The threshold check at line 50 is a pragmatic heuristic rather than a robust input validation.
- **Division by zero risk**: If a 100% markup on selling price is entered (R=100, converted to R=1.0 by line 60), line 220 performs `C/(1-1)`, i.e. division by zero, causing a runtime error. No guard is present for this edge case.

## Source Code

```
   3 REM %M%A%R%K%U%P
   4 PRINT TAB (5);"MARKUP CALCULATION"
   5 PRINT 
  10 PRINT "ENTER COST"
  20 INPUT C
  21 PRINT C
  30 PRINT "ENTER MARKUP DESIRED (PCT.)"
  40 INPUT R
  41 PRINT R
  50 IF R<1 THEN GOTO 120
  60 LET R=R/100
 120 PRINT "DESIRE M/U ON COST(C) OR SELL (S) PRICES"
 130 INPUT A$
 131 PRINT A$
 140 IF A$="C" THEN GOTO 180
 150 IF A$="S" THEN GOTO 220
 160 PRINT "INVALID CODE--TRY AGAIN"
 170 GOTO 120
 180 LET S=C+(C*R)
 190 LET S=INT (S*100+.5)/100
 200 LET B$="COST"
 210 GOTO 245
 220 LET S=C/(1-R)
 230 LET S=INT (S*100+.5)/100
 240 LET B$="SALES"
 245 PRINT 
 250 PRINT TAB (5);"WHEN COST IS $ ";C;" A MARKUP OF ";R*100;" PCT. ON "
 255 PRINT TAB (5);B$;" GIVES SALES OF $ ";S
 258 PRINT 
 260 PRINT "ANOTHER CALCULATION? Y/N"
 270 INPUT Q$
 280 IF Q$="Y" THEN GOTO 10
 290 STOP 
 300 SAVE "1011%7"
 310 LIST
```