You can get a good estimate of a monthly payment by using the expected monthly loan payment on a $1,000 loan as the base.  To use the table below: Locate the amount that corresponds to both the interest rate and number of months the loan is to be repaid (use the nearest interest rate).  Multiple that amount by the amount of the loan in thousands to get a close estimate of the monthly payment. 
Example: A $30,000 car loan with a 5% interest rate for 48 months:  $23.08 x 30
[30 = $30,000/1000] = a $692.40 monthly payment.
Interest
Rate

0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
12
Mths.

$83.33
$83.79
$84.25
$84.71
$85.17
$85.63
$86.09
$86.55
$87.01
$87.47
$87.92
$88.39
$88.85
$89.32
$89.79
$90.26
$90.74
$91.21
$91.68
$92.15
$92.62
$93.09

24
Mths.

$41.67
$42.12
$42.57
$43.02
$43.47
$43.92
$44.37
$44.82
$45.27
$45.72
$46.15
$46.41
$47.08
$47.55
$48.02
$48.49
$48.97
$49.45
$49.93
$50.41
$50.89
$51.37

36
Mths.

$27.78
$28.23
$28.68
$29.13
$29.58
$30.03
$30.48
$30.93
$31.38
$31.83
$32.27
$32.74
$33.22
$33.70
$34.18
$34.67
$35.16
$35.66
$36.16
$36.66
$37.16
$37.66
48
Mths.

$20.83
$21.28
$21.73
$22.18
$22.63
$23.08
$23.54
$24.00
$24.46
$24.92
$25.37
$25.85
$26.34
$26.83
$27.33
$27.84
$28.35
$28.86
$29.38
$29.90
$30.42
$30.92

60
Mths.

$16.67
$17.12
$17.57
$18.03
$19.49
$18.95
$19.41
$19.87
$20.33
$20.79
$21.25
$21.75
$22.25
$22.76
$23.27
$23.79
$24.32
$24.86
$25.40
$25.94
$26.48
$27.02

How to Calculate a Monthly Payment Without a Monthly Payment Calculator