In the market for a new calling card? We recommend buying a 'clean' calling card.

A 'clean' calling card is a calling card with relatively few extra fees. Because there are fewer fees, the pricing of clean calling cards is both easier to understand and more predictable under different usage scenarios.

With a clean card, you'll find that the advertised per minute rate is going to be closer to the 'real' per minute rate you end up paying after all extra fees are added in.

'Dirty' cards have more fees and it is harder to predict what you'll actually pay per minute. In some cases, the actual per minute rate you end up paying can be 5x or more higher than the advertised rate!

Since we are geeks, naturally we've calculated a 'clean card score' for 30 popular calling cards from 9 different companies. The results are shown in the tables below.

To calculate these scores, we did a detailed analysis of 9 pricing elements for each card. We then combined all these pricing factors with some assumptions about card usage (how many calls, how long the calls are, etc.) to determine an adjusted per minute rate and a clean score for each card.

The analysis produces two useful results. First is an estimate of the 'adjusted per minute rate' you can expect to pay for using a given card. This is, in effect, the loaded rate with all extra fees applied. The second result is the 'clean score'. The clean score calculated as the 'adjusted per minute rate' divided by the 'advertised per minute rate'. A lower score is better.

The lower the clean score, the closer the loaded rate is to the advertised rate. A higher score means a card has relatively more extra fees.

A perfect clean score would be 1.0 - meaning zero extra fees. But such cards don't exist because all cards charge at least a few extra fees.

In our analysis, the lowest scores we found were between 1.3 to 1.7. A clean score of 1.3 means the actual per minute rate is 1.3x more than the advertised rate -- meaning a card with a 1.0¢ per minute rate really costs you 1.3¢ per minute after fees are applied.

The highest score we found was 4.1 -- meaning a card with a 1.0¢ per minute rate really costs you 4.1¢ per minute after fees are applied. This is a pretty big difference! And we didn't even test the dirtiest cards on the market because those cards are so dirty we couldn't even do the calculations right.

NEXT: Key findings...