Spotrac and Overthecap give some different figures for the Seahawks' current cap space. They're both right, but neither is useful until you interpret it in terms of what happens once the season starts.
During the off-season, teams are allowed 70 or 90 or a gajillion players. All but 53 have to get cut before the season starts, so they won't actually earn those salaries, and so the current cap number (using all players under contract) is irrelevant.
Also during the off-season, teams only have to be under the salary cap for their top 51 (so that they can bring in 70 gajillion guys for training camp and pre-season, obviously). That number is also less-than-perfect because you actually have 53 on the active roster once the season starts.
Plus there's dead money, Practice Squad money (those players count against the cap), and a necessary buffer for replacing players on IR.
The Other Stuff
The Seahawks currently have $3,278,041 in dead money. That was easy.
Practice Squad players in 2015 received a minimum of $6,600 per week. Let's guesstimate $6,800 per week for 2016. Ten Players X 17 weeks X $6800 = $1,156,000 allocated for a full-strength Practice Squad.
Seattle ended 2015 with 10 players on IR. But it's important to note that when you replace an injured player, the salary-- and therefore cap charge-- of his replacement is pro-rated for the number of weeks that he's on the roster. So if you sign a replacement with just 3 weeks left in the regular season, his "annual salary" might be listed as $500,000, but he's only getting paid 3/17 of that amount (about $88,000).
In 2015, the Seahawks had 89 player weeks on IR or PUP (let's count both, because both will give you another player on the active roster to pay). That comes out to 89/17 = 5.24 full seasons; possibly a little less, because non-veteran players sometimes only get half pay while on IR.
So let's estimate 6 full seasons of replacement-level salary for IR. Those salaries should be about $500,000 (a veteran making slightly more in this situation should only count against the cap based on a lower number, thanks to a special rule). So we'll set aside $3 million for IR.
$3,278,041 Dead Money
$1,156,000 Practice Squad
$3,000,000 Injured Reserve/PUP replacements
Total Oddball Stuff: $7,434,041
The 53-man Roster
Spotrac has a total salary for just 38 players, which is irrelevant. OvertheCap shows a total salary for 50 players of $127,469,764, which is slightly more useful. That includes a dozen futures contracts in the $450K - $500K range.
Here are the estimated salaries/cap hits for the Seahawks' top draft picks:
Rd 1 Pick 26 = $1,578,989
Rd 2 Pick = $750,000
Rd 3 Pick = $600,000
Rd 3 Pick = $600,000
Rd 4 Pick = $550,000
Rd 5 Pick = $500,000
Now we're down to the same number as our guys on futures contracts, so the rest (as well as UDFA's) don't matter. All that matters is the guys who end out in the top 53.
Adding those 6 players to the 50 currently under contract gives us 56 players totaling $132,048,753. Now subtract out three players at the bottom ($450K each) and you get 53 players and $130,698,753.
And we're done.
2016 53-man roster salary: $130,698,753
Oddball Stuff: $7,434,041
Total Cap Spending: $138,132,794
Spotrac has a rather optimistic projection of the 2016 salary cap, which won't be announced until March. ESPN gives a range of $150M to $153.4M, which means the Seahawks have between $11.9M and $15.3M to spend on free agents and re-signing players.
Final caveat: Using that number, if you want to calculate what happens when we sign a player, be sure to subtract $500,000 from his 2016 cost (because he's replacing someone else). Likewise, if we cut a player, subtract $500,000 from his savings (because he has to be replaced).